user-accounts: Prevent usage of uninitialized value

gtk_widget_translate_coordinates() doesn't have to always set the
dest_x argument. Let's initialize the argument before use to prevent
usage of uninitialized value, which also prevents "Conditional jump
or move depends on uninitialised value(s)" messages from valgrind.

panels/user-accounts/cc-carousel.c

@@ -87,7 +87,7 @@ cc_carousel_item_get_x (CcCarouselItem *item,
 {
         GtkWidget *widget, *parent;
         gint width;
-        gint dest_x;
+        gint dest_x = 0;
 
         parent = GTK_WIDGET (carousel->stack);
         widget = GTK_WIDGET (item);