The Matroska muxer currently does not check the return value of
ff_isom_write_hvcc(), the function used to write mp4-style
HEVC-extradata as Matroska also uses it. This was intentionally done in
7a5356c7 to allow remuxing from mpeg-ts.

But if ff_isom_write_hvcc() fails, it has not output anything and the
file ends up without CodecPrivate and, if the input was Annex B, with
Annex B data, which is against the spec. So check the return value
again.

The underlying issue of not having extradata seems to have been fixed by
the introduction of the extract_extradata bitstream filter.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Reviewed-by: "mypopy@gmail.com" <mypopy@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>