avcodec/diracdec: Tighter checks on CODEBLOCKS_X/Y

Fixes very long but finite loop
Fixes: asan_heap-oob_107866c_42_041.drc
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
