shell: don't crash if a pointer's focus is null

It's possible for a pointer's focus to be null, e.g. because
the focus surface has been bestroyed. Prevent a crash when
that happens and a client takes too long to respond to a ping.

Signed-off-by: Emilio Pozuelo Monfort <emilio.pozuelo@collabora.co.uk>

src/shell.c

@@ -1866,7 +1866,8 @@ ping_timeout_handler(void *data)
 	shsurf->unresponsive = 1;
 
 	wl_list_for_each(seat, &shsurf->surface->compositor->seat_list, link)
-		if (seat->pointer->focus->surface == shsurf->surface)
+		if (seat->pointer->focus &&
+		    seat->pointer->focus->surface == shsurf->surface)
 			set_busy_cursor(shsurf, seat->pointer);
 
 	return 1;