8svx: Fixing header size, move decoding to per call instead of the first call.

The fate checksum change is due to the header size having been wrong.

Credit&Authorship for the code belongs to Justin Ruggles
Blame for bugs in this merging of the code belong to the Commiter
Commit message by Commiter
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>