Fix a crash when closing an X11 window with a selection
This was caused by weston_wm_handle_xfixes_selection_notify()
calling
weston_seat_set_selection()
with a NULL
source, apparently only
sometimes when closing an Xwayland window.
This was caused by weston_wm_handle_xfixes_selection_notify()
calling
weston_seat_set_selection()
with a NULL
source, apparently only
sometimes when closing an Xwayland window.