avcodec/vcr1: replace redundant checks from libav (8aba7968dd604aae91ee42cbce0be3dad7dceb30)  by asserts

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 164b67ca281fa5a47b965a858c7783aa547091b8)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/vcr1: return the actual number of consumed bytes

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit a4e70918316c6d1423e559aad15823a5e0453fcf)

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