Fix segfault when FB_ID isn't set

This would previously result in a crash:

    ==169727==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000028 (pc 0x7f3e0c4d7dc7 bp 0x7ffc97f8aff0 sp 0x7ffc97f8afc0 T0)
    ==169727==The signal is caused by a READ memory access.
    ==169727==Hint: address points to the zero page.
        #0 0x7f3e0c4d7dc6 in layer_update_priority ../subprojects/libliftoff/layer.c:152
        #1 0x7f3e0c4d3a4c in update_layers_priority ../subprojects/libliftoff/alloc.c:573
        #2 0x7f3e0c4d3d7f in liftoff_output_apply ../subprojects/libliftoff/alloc.c:590
        …

layer.c

@@ -149,7 +149,7 @@ void layer_update_priority(struct liftoff_layer *layer, bool make_current) {
 	/* TODO: also bump priority when updating other
 	 * properties */
 	prop = layer_get_property(layer, "FB_ID");
-	if (prop->changed) {
+	if (prop != NULL && prop->changed) {
 		layer->pending_priority++;
 	}