avcodec/wmaprodec: Check block_align

Fixes undefined behavior
Its not known if this condition can be triggered through crafted input.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
imc: Catch a division by zero

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