avcodec/mpegvideo: fix ac/dc_val and coded_block table sizes

With interlaced vc1 it was possible that accesses could happen outside these
tables before this.

Regression since 017e234c204f8ffb5f85a073231247881be1ac6f
Reproduced with a sample from Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Fixes (again) : 480i30__codec_WVC1__mode_2__framerate_29.970__type_2__preproc_17.SIGFPE.bfa.390.wmv
No releases are affected by this

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>