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>
