avutil/utils: check and warn about llrint() brokenness

Such broken llrint() exist for example on netbsd

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/vc1dec: Check source picture availability in vc1_mc_4mv_chroma4()

Fixes null pointer dereference

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Document "cache" protocol

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>