kiosk-shell: Don't assume a valid output

Turns out kiosk_shell_output_set_active_surface_tree() requires having a
valid output which as seen in the wild might not be case. Prevents
an illegal dereference on an invalid shoutput.

Fixes: #920
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

kiosk-shell/kiosk-shell.c

@@ -963,6 +963,7 @@ desktop_surface_removed(struct weston_desktop_surface *desktop_surface,
 						     WESTON_ACTIVATE_FLAG_NONE);
 		} else {
 			kiosk_seat->focused_surface = NULL;
+			if (shoutput)
 				kiosk_shell_output_set_active_surface_tree(shoutput,
 									   NULL);
 		}