Results of IDCT can by far outreach the range of ff_cropTbl[], leading
to overreads and potentially crashes.
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
"Copyright (c) 2001 Michael Niedermayer" and "part of Libav" is not likely
not only am i not a libav developer there also was no libav in 2001
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>