lzo: Handle integer overflow

get_len can overflow for specially crafted payload.

Reported-By: Don A. Baley <donb@securitymouse.com>
CC: libav-stable@libav.org
(cherry picked from commit ccda51b14c0fcae2fad73a24872dce75a7964996)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>