user-accounts: Fix use of an uninitialised variable in the carousel

`dest_x` is not set if `gtk_widget_translate_coordinates()` fails, which
it can do before the widget is realised.

This fixes a valgrind warning, but doesn’t change any user-visible
behaviour as far as I can tell.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

panels/user-accounts/cc-carousel.c

@@ -93,12 +93,13 @@ cc_carousel_item_get_x (CcCarouselItem *item,
         widget = GTK_WIDGET (item);
 
         width = gtk_widget_get_allocated_width (widget);
-        gtk_widget_translate_coordinates (widget,
-                                          parent,
-                                          width / 2,
-                                          0,
-                                          &dest_x,
-                                          NULL);
+        if (!gtk_widget_translate_coordinates (widget,
+                                               parent,
+                                               width / 2,
+                                               0,
+                                               &dest_x,
+                                               NULL))
+                return 0;
 
         return CLAMP (dest_x - ARROW_SIZE,
                       0,