matroskadec: Fix a double negation typo

This typo has existed since this code was added in c16582579.
Newer versions of clang pointed out that this comparison always
was true (since the result of the negation is either 0 or 1, while
AVDISCARD_ALL has the value 48).

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/matroskadec.c

@@ -2560,7 +2560,7 @@ 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);