Popover: Clarify/guard out rect of get_pointing_to

Popover: Clarify/guard out rect of get_pointing_to

Clarify that we zero out the widget coords and only keep its dimensions.

If we have no widget to fall-back to, memset to 0 the output @rect since
we return FALSE whether or not we have widget, so protect users from not
knowing if there was a widget and possibly accessing uninitialised ints.

Bug: https://gitlab.gnome.org/GNOME/gtk/-/issues/893#note_1766079
Origin: 3.24.39, commit:a6d40b610be6acb635d8732c11bb6d866bf2e95d

Gbp-Pq: Name Popover-Clarify-guard-out-rect-of-get_pointing_to.patch