lavf/mkv: avoid negative ts by default.

This fixes playback in some circumstances (like webm in firefox).
Regression after 2c34367b.

It is also matching the Matroska specifications:
http://matroska.org/technical/specs/notes.html, "The quick eye will
notice that if a Cluster's Timecode is set to zero, it is possible to
have Blocks with a negative Raw Timecode. Blocks with a negative Raw
Timecode are not valid."
