avformat/matroskaenc: Fix writing of markers

When the marker writing code was merged from libav to FFmpeg
in dc62016c, it failed to take into account that the meaning of
cluster_pos had changed in bda5b662; in particular, the special
value for “I'm not currently working on a cluster” had changed
from 0 to -1. This makes the avio_write_marker() call never
be called. Update the if statement to fix it.

Fixes: Ticket9843
Signed-off-by: Steinar H. Gunderson <steinar+ffmpeg@gunderson.no>
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/matroskaenc.c

@@ -3027,7 +3027,7 @@ static int mkv_write_packet(AVFormatContext *s, const AVPacket *pkt)
         }
     }
 
-    if (!mkv->cluster_pos)
+    if (mkv->cluster_pos == -1)
         avio_write_marker(s->pb,
                           av_rescale_q(pkt->dts, s->streams[pkt->stream_index]->time_base, AV_TIME_BASE_Q),
                           keyframe && (mkv->have_video ? codec_type == AVMEDIA_TYPE_VIDEO : 1) ? AVIO_DATA_MARKER_SYNC_POINT : AVIO_DATA_MARKER_BOUNDARY_POINT);