libavformat/matroskaenc.c: fix small memory leaks on error

Fixing small leaks that can occur when mkv_write_tracks fails in mkv_write_header
(e.g., if video track has unknown codec). Also changing mkv_write_seekhead to take
the MatroskaMuxContext to avoid having dangling pointers.

Signed-off-by: Neil Birkbeck <neil.birkbeck@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
