avcodec/mss2: Fix integer overflow

This also simplifies the code
Fixes: signal_sigabrt_7ffff6ac8cc9_2943_cov_3588637614_mss2_speech.wmv

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