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
avutil/lzo: add asserts to be double sure against overflows

These asserts cannot fail since d6af26c55c1ea30f85a7d9edbc373f53be1743ee

Based-on: ccda51b14c0fcae2fad73a24872dce75a7964996
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit 'ccda51b14c0fcae2fad73a24872dce75a7964996'

* commit 'ccda51b14c0fcae2fad73a24872dce75a7964996':
  lzo: Handle integer overflow

Conflicts:
	libavutil/lzo.c

This is basically not merging changes due to this being fixed differently
in FFmpeg

See: d6af26c55c1ea30f85a7d9edbc373f53be1743ee
See: cf2b7c01f81c1fb3283a1390c0ca9a2f81f4f4a8
Merged-by: Michael Niedermayer <michaelni@gmx.at>