matroskadec: fix double negation typo

Fixes CID602563
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskadec.c

@@ -2398,7 +2398,7 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
         tracks[i].audio.buf_timecode = AV_NOPTS_VALUE;
         tracks[i].end_timecode = 0;
         if (tracks[i].type == MATROSKA_TRACK_TYPE_SUBTITLE
-            && !tracks[i].stream->discard != AVDISCARD_ALL) {
+            && tracks[i].stream->discard != AVDISCARD_ALL) {
             index_sub = av_index_search_timestamp(tracks[i].stream, st->index_entries[index].timestamp, AVSEEK_FLAG_BACKWARD);
             if (index_sub >= 0
                 && st->index_entries[index_sub].pos < st->index_entries[index_min].pos