avcodec/diracdec: Use 64bit in calculation of codeblock coordinates

Fixes integer overflow
Fixes out of array read
Fixes: asan_heap-oob_107866c_42_041.drc
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
