demux: fix a corner case

demux_read_any_packet() attempts to call read_packet(), but if no stream
is active, it can decide not to read anything. The function will return
NULL, which implies EOF. Fix this by explicitly
setting demux_stream->active if needed.

Also use dequeue_packet() instead of demux_read_packet(), because it's
cleaner. (Shouldn't change behavior.)

Possibly fixes #938.

demux/demux.c

@@ -536,8 +536,10 @@ struct demux_packet *demux_read_any_packet(struct demuxer *demuxer)
     for (int retry = 0; retry < 2; retry++) {
         for (int n = 0; n < demuxer->num_streams; n++) {
             struct sh_stream *sh = demuxer->streams[n];
-            if (demux_has_packet(sh))
-                return demux_read_packet(sh);
+            sh->ds->active = sh->ds->selected; // force read_packet() to read
+            struct demux_packet *pkt = dequeue_packet(sh->ds);
+            if (pkt)
+                return pkt;
         }
         // retry after calling this
         pthread_mutex_lock(&demuxer->in->lock);