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>
