desktop-shell: Guard against invalid seats in get_shell_seat()

Starting with commit 'desktop-shell: Embed keyboard focus handle code
when activating' we are iterating over all the seats when removing a
weston_desktop_surface to be able to invalidate the activate surface.

Upon compositor tear down the shell destroy might invalidate seats
before removal of the weston_desktop_surface making the shell_seat
itself invalid. This wasn't apparent at that time because we're not
handling at that the removal of surfaces from layers.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

desktop-shell/shell.c

@@ -2203,8 +2203,12 @@ get_shell_seat(struct weston_seat *seat)
 {
 	struct wl_listener *listener;
 
+	if (!seat)
+		return NULL;
+
 	listener = wl_signal_get(&seat->destroy_signal, destroy_shell_seat);
-	assert(listener != NULL);
+	if (!listener)
+		return NULL;
 
 	return container_of(listener,
 			    struct shell_seat, seat_destroy_listener);
@@ -2340,7 +2344,7 @@ desktop_surface_removed(struct weston_desktop_surface *desktop_surface,
 		 * removed) surface when attempting to de-activate it. It will
 		 * also update the focused_surface once it has a chance to run.
 		 */
-		if (surface == shseat->focused_surface)
+		if (shseat && surface == shseat->focused_surface)
 			shseat->focused_surface = NULL;
 	}
 
@@ -3760,13 +3764,14 @@ activate(struct desktop_shell *shell, struct weston_view *view,
 
 	weston_view_activate_input(view, seat, flags);
 
-	if (shseat->focused_surface) {
+	if (shseat && shseat->focused_surface) {
 		struct shell_surface *current_focus =
 			get_shell_surface(shseat->focused_surface);
 		assert(current_focus);
 		shell_surface_deactivate(current_focus);
 	}
 
+	if (shseat)
 		shseat->focused_surface = main_surface;
 	shell_surface_activate(shsurf);