avformat/matroskaenc: fix cue relative position values when CRC32 is enabled

The dynamic buffer does not contain the CRC32 element so calls to avio_tell()
don't take it into account. This resulted in CueRelativePosition values being
six bytes short.
This is a regression since 6724525a1576ca334d2ffdc085620bb44aea7394

Instead of adding yet another custom check for CRC32 to fix a size or an offset,
remove the existing ones and reserve the six bytes in the dynamic buffer.

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