asv1: Verify the amount of extradata

The init function reads one byte of extradata.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit f50803354c6acb4575379d7c54ca48ec5d36dd61)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>