Mark the airplay_gid free by nulling it.

player.c

@@ -1705,8 +1705,10 @@ void player_thread_cleanup_handler(void *arg) {
   if (conn->stream.type == ast_apple_lossless)
     terminate_decoders(conn);
 
-  if (conn->airplay_gid)
+  if (conn->airplay_gid) {
     free(conn->airplay_gid);
+    conn->airplay_gid = NULL;
+  }
 
   reset_anchor_info(conn);
   release_play_lock(conn);