demux_lavf: fix null derefence of stream variable

demux_stream_is_selected() already checks if stream is valid, so move
the image format hack check after that.

Fixes: 4ef7931

demux/demux_lavf.c

@@ -1260,15 +1260,15 @@ static bool demux_lavf_read_packet(struct demuxer *demux,
  struct sh_stream *stream = info->sh;
  AVStream *st = priv->avfc->streams[pkt->stream_index];
 
- // Never send additional frames for streams that are a single frame.
- if (stream->image && priv->format_hack.first_frame_only && pkt->pos != 0) {
+ if (!demux_stream_is_selected(stream)) {
  av_packet_unref(pkt);
- return true;
+ return true; // don't signal EOF if skipping a packet
  }
 
- if (!demux_stream_is_selected(stream)) {
+ // Never send additional frames for streams that are a single frame.
+ if (stream->image && priv->format_hack.first_frame_only && pkt->pos != 0) {
  av_packet_unref(pkt);
- return true; // don't signal EOF if skipping a packet
+ return true;
  }
 
  struct demux_packet *dp = new_demux_packet_from_avpacket(pkt);