vcr1: add sanity checks

Fixes invalid reads with corrupted files.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
pictordec: break out of both decoding loops when y drops below 0

Otherwise picmemset can get called with negative y, resulting in an
invalid write.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
pictordec: pass correct context to avpriv_request_sample

Fixes invalid reads.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
lavf: move a variable declaration to the block where it's used
lavf: avoid integer overflow when estimating bitrate

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
4xm: check that bits per sample is strictly positive

Avoids a divide by zero.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
ape demuxer: check for EOF in potentially long loops

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org