shared/bap: Make bt_bap_ref_safe check if the instance is attached

This makes bt_bap_ref_safe check that bap instance is part of sessions
list which means it is attached/valid.

src/shared/bap.c

@@ -1033,7 +1033,7 @@ static void stream_notify_metadata(struct bt_bap_stream *stream)
 
 static struct bt_bap *bt_bap_ref_safe(struct bt_bap *bap)
 {
-	if (!bap || !bap->ref_count)
+	if (!bap || !bap->ref_count || !queue_find(sessions, NULL, bap))
 		return NULL;
 
 	return bt_bap_ref(bap);