avformat/matroskaenc: Don't segfault when seekability changes

avformat/matroskaenc: Don't segfault when seekability changes

If the Matroska muxer's AVIOContext was unseekable when writing the
header, but is seekable when writing the trailer, the code for writing
the trailer presumes that a dynamic buffer exists and tries to update
its content in order to overwrite data that has already been
preliminarily written when writing the header, yet said buffer doesn't
exist as it has been written finally and not preliminarily when writing
the header (because of the unseekability it was presumed that one won't
be able to update the data anyway).

This commit adds a check for this and also for a similar situation
involving updating extradata with new data from packet side-data.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>