demux: fix backward playback at EOF with full demuxer cache

Fixes "mpv file.mkv --cache --demuxer-cache-wait --play-dir=backward",
and other situations where the demuxer cache contains the entire file,
and playback is to start from the end. It also can be triggered when
starting playback normally with --cache, and once everything is in the
cache, enabling backward playback and seeking past EOF.

In all cases, the cache seek will set reader_head=NULL (because you
seeked to/past EOF). Then the code (the one modified by this commit)
sees that ds->queue->is_bof==true, and thinks we've reached BOF
(beginning of file) while searching for a useful packet, i.e. we found
nothing and playback really can only end.

Obviously this is nonsense, we've found only nothing if we actually
searched from the beginning, not some "random" reader_head (== first)
value that does not include the entire cache. That means the condition
should trigger only if the start of the search (first variable) points
to the beginning of the cache (ds->queue->head).

Not taking this if means we'll seek to an earlier position and retry.
Also, a seek before the beginning of the cache will always end up with
reader_head==ds->queue->head, i.e. we'll terminate properly.

That comment was quite right.

demux/demux.c

@@ -1392,9 +1392,7 @@ static void find_backward_restart_pos(struct demux_stream *ds)
     }
 
     if (!target) {
-        // Note: assume this holds true. You could think of various reasons why
-        // this might break.
-        if (ds->queue->is_bof) {
+        if (ds->queue->is_bof && first == ds->queue->head) {
             MP_VERBOSE(in, "BOF for stream %d\n", ds->index);
             ds->back_restarting = false;
             ds->back_range_started = false;