mkv_write_seekhead() would up until now try to seek to the position where
the SeekHead ought to be written, write the SeekHead and seek back. The
first of these seeks was checked as was writing, yet the seek back was
unchecked. Moreover the return value of mkv_write_seekhead() was unchecked
(the ordinary return value was the position where the SeekHead was written).

This commit changes this: Everything is checked. In the unseekable case
(where the first seek may nevertheless work when it happens in the buffer)
a failure at the first seek is not considered an error. In any case,
failure to seek back is an error.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>