player: don't even start loading external tracks when aborting loading

It seems the ytdl script like to continue loading external tracks even
if loading was aborted. Trying to do so will still quickly fail, but not
without a load of log noise. So check and error out early.

player/loadfile.c

@@ -582,7 +582,7 @@ int mp_add_external_file(struct MPContext *mpctx, char *filename,
                          enum stream_type filter, bool unlock)
 {
     struct MPOpts *opts = mpctx->opts;
-    if (!filename)
+    if (!filename || mp_cancel_test(mpctx->playback_abort))
         return -1;
 
     char *disp_filename = filename;