avcodec/vc1dec: field pictures with direct mode MBs, followed by frame pictures are not supported

This case could occur when cuting and concatenating bitstreams

Fixes out of array read
Fixes: asan_heap-oob_1b33fdd_2849_cov_478905890_SA10143.vc1
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
