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.
2 jobs for fix-crash-on-x11-window-close in 1 minute and 22 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Container Prep
passed #507442
container_prep

00:00:30

 
  Build
passed #507443
build-native-meson

00:00:52