xwm: check whether the picked seat can be NULL

The seat picked in weston_wm_window_handle_moveresize can sometimes
be NULL when it is (somehow) triggered with all buttons released.

This patch checks whether the seat is NULL to avoid NULL dereference.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=80837
Signed-off-by: Boyan Ding <stu_dby@126.com>
1 file changed