Frame::IsValid: fix track_number check

previously IsValidTrackNumber() expected a track_number > 0
this also fixes a warning:
'comparison of unsigned expression < 0 is always false'

Change-Id: I839863986ff776aae59e38ee7e4a1a710081303a

mkvmuxer.cpp

@@ -201,7 +201,7 @@ bool Frame::IsValid() const {
       (additional_ != NULL && additional_length_ == 0)) {
     return false;
   }
-  if (track_number_ < 0 || track_number_ > kMaxTrackNumber) {
+  if (track_number_ == 0 || track_number_ > kMaxTrackNumber) {
     return false;
   }
   return true;