wayland/pointer-gestures: Fix crash in get_*_gesture after pointer is destroyed

Properly handle cases where the pointer object has already been
destroyed to prevent the compositor from crashing with a null pointer
dereference when a client invokes a get_*_gesture method after the
pointer object is destroyed e.g. when the current login session is not on
foreground.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/2237
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2690>

(cherry picked from commit 992e169d)