avcodec/pcm: Fix overflow in bitrate computation

Fixes: 129ca3e28d73af7b1e24a9d4118e7a2d/signal_sigabrt_7ffff6ae7cc9_836_762b310fc3ef6087bd7771e5d8e90b9b.asf

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
