Fix return value, invalid pointer to integer conversion with clang 15.

Index: src/libmpd-playlist.c
--- src/libmpd-playlist.c.orig
+++ src/libmpd-playlist.c
@@ -780,7 +780,7 @@ int mpd_playlist_load(MpdObj *mi, const char *path)
 	if(mpd_lock_conn(mi))
 	{
 		debug_printf(DEBUG_ERROR,"lock failed\n");
-		return NULL;
+		return MPD_LOCK_FAILED;
 	}
     mpd_sendLoadCommand(mi->connection,path);
 	mpd_finishCommand(mi->connection);