mvdec: minor simplification, remove av_inv_q()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
mvdec: check channel count.

Fixes division by 0

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