Ignore xing number of frames if the file is larger than indicated.

Fixes ticket #3777.
Merge remote-tracking branch 'cehoyos/master'

* cehoyos/master:
  Ignore xing number of frames if the file is larger than indicated.

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avformat/matroskaenc: Factor mkv_start_new_cluster() out

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/matroskaenc: Start new cluster if relative timestamp could not otherwise be stored

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>