general: Fix various strict-aliasing warnings with g_clear_pointer()

This is just unfortunate. It’s an aliasing violation to cast a pointer
to a pointer (and there’s no way round that), although in practice it
will not cause a problem. People do quite often compile with
`-Werror=strict-aliasing`, though, so fixing the warnings is helpful.

Warnings are of the form:
```
../../source/gnome-control-center/panels/keyboard/cc-keyboard-shortcut-dialog.c: In function ‘cc_keyboard_shortcut_dialog_finalize’:
../../source/gnome-control-center/panels/keyboard/cc-keyboard-shortcut-dialog.c:518:20: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  518 |   g_clear_pointer ((GtkWindow**)&self->shortcut_editor, gtk_window_destroy);
/opt/gnome/install/include/glib-2.0/glib/gmacros.h:871:47: note: in definition of macro ‘G_STATIC_ASSERT’
  871 | #define G_STATIC_ASSERT(expr) _Static_assert (expr, "Expression evaluates to false")
      |                                               ^~~~
../../source/gnome-control-center/panels/keyboard/cc-keyboard-shortcut-dialog.c:518:3: note: in expansion of macro ‘g_clear_pointer’
  518 |   g_clear_pointer ((GtkWindow**)&self->shortcut_editor, gtk_window_destroy);
      |   ^~~~~~~~~~~~~~~
In file included from /opt/gnome/install/include/glib-2.0/glib/gatomic.h:30,
                 from /opt/gnome/install/include/glib-2.0/glib/gthread.h:34,
                 from /opt/gnome/install/include/glib-2.0/glib/gasyncqueue.h:34,
                 from /opt/gnome/install/include/glib-2.0/glib.h:34:
../../source/gnome-control-center/panels/keyboard/cc-keyboard-shortcut-dialog.c:518:20: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  518 |   g_clear_pointer ((GtkWindow**)&self->shortcut_editor, gtk_window_destroy);
/opt/gnome/install/include/glib-2.0/glib/glib-typeof.h:39:36: note: in definition of macro ‘glib_typeof’
   39 | #define glib_typeof(t) __typeof__ (t)
      |                                    ^
../../source/gnome-control-center/panels/keyboard/cc-keyboard-shortcut-dialog.c:518:3: note: in expansion of macro ‘g_clear_pointer’
  518 |   g_clear_pointer ((GtkWindow**)&self->shortcut_editor, gtk_window_destroy);
      |   ^~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
```

I believe it’s better to fix these by expanding out the
`g_clear_pointer()` call, than by changing the types of variables — the
latter approach means everything becomes a `GtkWidget` or a `GtkWindow`,
which loses type specificity. So this approach is in contrast to that
taken in commit 1bafd46ea3, for example.

Alternative approaches would be:
 1. Add internal `cc_clear_window()` and `cc_clear_widget()` helpers
    which do this in a single line without aliasing violations.
 2. Enforce compiling with `-Wno-strict-aliasing` if strict aliasing is
    not something that g-c-c maintainers want to care about (which would
    be fine, aliasing checks probably won’t catch any bugs in this kind
    of code).

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Fixes: #2563

panels/color/cc-color-panel.c

@@ -1848,7 +1848,11 @@ cc_color_panel_dispose (GObject *object)
   g_clear_object (&self->list_box_size);
   g_clear_pointer (&self->sensors, g_ptr_array_unref);
   g_clear_pointer (&self->list_box_filter, g_free);
-  g_clear_pointer ((GtkWindow **)&self->dialog_assign, gtk_window_destroy);
+
+  if (self->dialog_assign != NULL) {
+    gtk_window_destroy (GTK_WINDOW (self->dialog_assign));
+    self->dialog_assign = NULL;
+  }
 
   G_OBJECT_CLASS (cc_color_panel_parent_class)->dispose (object);
 }

panels/common/cc-vertical-row.c

@@ -189,7 +189,10 @@ cc_vertical_row_dispose (GObject *object)
   }
 
   cc_vertical_row_set_activatable_widget (self, NULL);
-  g_clear_pointer ((GtkWidget**)&priv->header, gtk_widget_unparent);
+  if (priv->header != NULL) {
+    gtk_widget_unparent (GTK_WIDGET (priv->header));
+    priv->header = NULL;
+  }
 
   G_OBJECT_CLASS (cc_vertical_row_parent_class)->dispose (object);
 }

panels/keyboard/cc-keyboard-shortcut-dialog.c

@@ -515,7 +515,11 @@ cc_keyboard_shortcut_dialog_finalize (GObject *object)
   g_clear_pointer (&self->search_terms, g_strfreev);
   g_clear_object (&self->sections);
   g_clear_object (&self->filtered_shortcuts);
-  g_clear_pointer ((GtkWindow**)&self->shortcut_editor, gtk_window_destroy);
+
+  if (self->shortcut_editor != NULL) {
+    gtk_window_destroy (GTK_WINDOW (self->shortcut_editor));
+    self->shortcut_editor = NULL;
+  }
 
   G_OBJECT_CLASS (cc_keyboard_shortcut_dialog_parent_class)->finalize (object);
 }

panels/network/connection-editor/ce-page-8021x-security.c

@@ -154,9 +154,13 @@ ce_page_8021x_security_dispose (GObject *object)
 	CEPage8021xSecurity *self = CE_PAGE_8021X_SECURITY (object);
 
         g_clear_object (&self->connection);
-        g_clear_pointer ((GtkWidget **) &self->security, gtk_widget_unparent);
         g_clear_object (&self->group);
 
+        if (self->security != NULL) {
+                gtk_widget_unparent (GTK_WIDGET (self->security));
+                self->security = NULL;
+        }
+
 	G_OBJECT_CLASS (ce_page_8021x_security_parent_class)->dispose (object);
 }
 

panels/privacy/bolt/cc-bolt-page.c

@@ -936,7 +936,11 @@ cc_bolt_page_dispose (GObject *object)
 
   /* Must be destroyed in dispose, not finalize. */
   cc_bolt_device_dialog_set_device (self->device_dialog, NULL, NULL);
-  g_clear_pointer ((GtkWindow **) &self->device_dialog, gtk_window_destroy);
+
+  if (self->device_dialog != NULL) {
+    gtk_window_destroy (GTK_WINDOW (self->device_dialog));
+    self->device_dialog = NULL;
+  }
 
   G_OBJECT_CLASS (cc_bolt_page_parent_class)->dispose (object);
 }

panels/sound/cc-volume-slider.c

@@ -204,8 +204,14 @@ cc_volume_slider_dispose (GObject *object)
 {
   CcVolumeSlider *self = CC_VOLUME_SLIDER (object);
 
-  g_clear_pointer ((GtkWidget **) &self->volume_scale, gtk_widget_unparent);
-  g_clear_pointer ((GtkWidget **) &self->mute_button, gtk_widget_unparent);
+  if (self->volume_scale != NULL) {
+    gtk_widget_unparent (GTK_WIDGET (self->volume_scale));
+    self->volume_scale = NULL;
+  }
+  if (self->mute_button != NULL) {
+    gtk_widget_unparent (GTK_WIDGET (self->mute_button));
+    self->mute_button = NULL;
+  }
 
   g_clear_object (&self->mixer_control);
   g_clear_object (&self->stream);