avcodec/vp3: pass correct context to av_log()

Fixes null pointer dereference
Fixes: signal_sigsegv_000_3694_cov_1050071691_theora.mkv

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