avformat/matroskadec: Fix handling gigantic durations

matroska_parse_block currently asserts that the duration is not equal to
AV_NOPTS_VALUE, but there is nothing that actually guarantees this. It
is easy to create (spec-compliant) files which run into this assert;
so replace it and instead cap the duration to INT64_MAX, as the duration
field of an AVPacket is an int64_t.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit 3714d452b8)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/matroskadec.c

@@ -3331,7 +3331,8 @@ static int matroska_parse_block(MatroskaDemuxContext *matroska, AVBufferRef *buf
     st = track->stream;
     if (st->discard >= AVDISCARD_ALL)
         return res;
-    av_assert1(block_duration != AV_NOPTS_VALUE);
+    if (block_duration > INT64_MAX)
+        block_duration = INT64_MAX;
 
     block_time = sign_extend(AV_RB16(data), 16);
     data      += 2;