compositor-wayland: Ignore pointer enter on destroyed surface

compositor-wayland: Ignore pointer enter on destroyed surface

Due to race conditions, it is (vanishingly unlikely but) possible to
receive a wl_pointer.enter event referring to a wl_surface we have just
destroyed. If this happens, wl_surface will be NULL. Detect this, clear
out our focus, and return.

Other pointer and keyboard events are robust against destroyed surfaces.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Cc: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
[Pekka: remove call to input_set_cursor()]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>