asfenc: return error on negative timestamp

According to the specification the timestamp is represented by a 32bit
unsigned.

CC: libav-stable@libav.org
iff: validate CMAP palette size

Fixes CVE-2013-2495

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

CC: libav-stable@libav.org