avcodec/vp3: Fix "runtime error: left shift of negative value"

Fixes: 5c6129154b356b80bcab86f9e3ee5d29/signal_sigabrt_7ffff6ae7cc9_7322_d26ac6d7cb6567db1b8be0159b387d0b.ogg

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