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."

libavformat/matroskaenc.c

@@ -902,6 +902,9 @@ static int mkv_write_header(AVFormatContext *s)
     if (!strcmp(s->oformat->name, "webm")) mkv->mode = MODE_WEBM;
     else                                   mkv->mode = MODE_MATROSKAv2;
 
+    if (s->avoid_negative_ts < 0)
+        s->avoid_negative_ts = 1;
+
     mkv->tracks = av_mallocz(s->nb_streams * sizeof(*mkv->tracks));
     if (!mkv->tracks)
         return AVERROR(ENOMEM);