loadfile: fix --loop-playlist=N with --prefetch-playlist

With --prefetch-playlist, mp_next_file() is called continously since the
last second of playback or when viewing an image, which decreases
--loop-playlist=N to 1.

Fix this by returning before calling mp_next_file() if prefetching
already started.

Author: guidocella

player/loadfile.c

@@ -1250,11 +1250,11 @@ static void open_demux_reentrant(struct MPContext *mpctx)
 
 void prefetch_next(struct MPContext *mpctx)
 {
-    if (!mpctx->opts->prefetch_open)
+    if (!mpctx->opts->prefetch_open || mpctx->open_active)
         return;
 
     struct playlist_entry *new_entry = mp_next_file(mpctx, +1, false);
-    if (new_entry && !mpctx->open_active && new_entry->filename) {
+    if (new_entry && new_entry->filename) {
         MP_VERBOSE(mpctx, "Prefetching: %s\n", new_entry->filename);
         start_open(mpctx, new_entry->filename, new_entry->stream_flags, true);
     }