In gtk_container_real_set_focus_child(), we try to scroll to the
position of the new :focus-child if we have h or v adjustments.

gtk_widget_translate_coordinates() returns FALSE if neither widget is
realized or in other situations that cause output parameters x and y not
to be set. Thus, if the caller did not initialise x/y and uses them even
if the function returns FALSE, they are using uninitialised variables.

In gtk_container_real_set_focus_child(), we did not check the return
value but merrily went ahead and used x and y regardless. This is UB, as
caught by Valgrind, as well as being pointless.

The trivial fix is to exit early if (!gtk_widget_translate_coordinates).

https://bugzilla.gnome.org/show_bug.cgi?id=776909