libavformat/matroskadec: fix unsigned overflow to improve seeking

When seeking a file where codec delay is greater than 0, the timecode
can become negative after offsetting by the codec delay. Failing to cast
to a signed int64 will cause the check against skip_to_timecode to evaluate
true for these negative values. This breaks the "skip_to" seek mechanism.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/matroskadec.c

@@ -3150,7 +3150,10 @@ static int matroska_parse_block(MatroskaDemuxContext *matroska, uint8_t *data,
 
     if (matroska->skip_to_keyframe &&
         track->type != MATROSKA_TRACK_TYPE_SUBTITLE) {
-        if (timecode < matroska->skip_to_timecode)
+        // Compare signed timecodes. Timecode may be negative due to codec delay
+        // offset. We don't support timestamps greater than int64_t anyway - see
+        // AVPacket's pts.
+        if ((int64_t)timecode < (int64_t)(matroska->skip_to_timecode))
             return res;
         if (is_keyframe)
             matroska->skip_to_keyframe = 0;