loadfile: fix --loop-playlist + --shuffle + --prefetch-playlist

With this combination of options, the playlist is shuffled every time
the last playlist is reached, instead of when restarting from the first
playlist entry. Abort prefetching in this case, as we can't predict
which file to prefetch until the playlist is shuffled when actually
going to the first file again.

player/loadfile.c

@@ -1935,8 +1935,11 @@ struct playlist_entry *mp_next_file(struct MPContext *mpctx, int direction,
         next->playlist_prev_attempt = true;
     if (!next && mpctx->opts->loop_times != 1) {
         if (direction > 0) {
-            if (mpctx->opts->shuffle)
+            if (mpctx->opts->shuffle) {
+                if (!update_loop)
+                    return NULL;
                 playlist_shuffle(mpctx->playlist);
+            }
             next = playlist_get_first(mpctx->playlist);
             if (next && mpctx->opts->loop_times > 1 && update_loop) {
                 mpctx->opts->loop_times--;