xwayland: Make sure we have a focus window

During the InitInput() phase, the wayland events get dequeued so we
can possibly end up calling dispatch_pointer_motion_event().

If this occurs before xwl_seat->focus_window is set, it leads to a NULL
pointer derefence and a segfault.

Check for xwl_seat->focus_window in both pointer_handle_frame() and
relative_pointer_handle_relative_motion() prior to calling
dispatch_pointer_motion_event()  like it's done in
pointer_handle_motion().

Bugzilla: https://bugzilla.redhat.com/1410804


Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 8c9909a9)