avutil/lzo: Fix integer overflow

Embargoed-till: 2014-06-27 requested by researcher, but embargo broken by libav today (git and mailing list)

Fixes: LMS-2014-06-16-4
Found-by: "Don A. Bailey" <donb@securitymouse.com>
See: ccda51b14c0fcae2fad73a24872dce75a7964996
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
