lavf/tta: check header and seek table checksum

Since we don't check for seek table integrity in the decoder anymore,
check it in the demuxer instead (Only tta files are guaranteed to have
one, and it should be valid there).

Check also for header integrity, since the check done in the decoder
is ignored because matroska doesn't store the header at all.

Signed-off-by: James Almer <jamrial@gmail.com>
