demux_mkv: avoid an error message in a corner case

If --demuxer-mkv-probe-start-time=no is used, and a seek is triggered on
start, then cluster_start will be 0, and the packet reading code will
print an error message about not finding valid data. This fixes itself
since it invokes the resync code, but it's still pretty ugly. Avoid this
by always initializing cluster_start.

demux/demux_mkv.c

@@ -1965,6 +1965,7 @@ static int demux_mkv_open(demuxer_t *demuxer, enum demux_check check)
         }
         if (id == MATROSKA_ID_CLUSTER) {
             MP_VERBOSE(demuxer, "|+ found cluster\n");
+            mkv_d->cluster_start = start_pos;
             break;
         }
         int res = read_header_element(demuxer, id, start_pos);