Online accounts: avoid an extraneous unref

When bailing out early, the cleanup path is unreffing the provider
because the variable was used in the loop. We need to set it back
to NULL there.

https://bugzilla.gnome.org/show_bug.cgi?id=657819

panels/online-accounts/cc-online-accounts-panel.c

@@ -479,6 +479,7 @@ on_toolbar_add_button_clicked (GtkToolButton *button,
                                  goa_provider_get_provider_type (provider),
                                  provider_name);
       g_free (provider_name);
+      provider = NULL;
     }
   gtk_combo_box_set_active (GTK_COMBO_BOX (combo_box), 0);