The original code left-shifts negative values, which is undefined
in the C99 specification (the one used during normal Libav compilation).
This change multiplies by (1 << shift), which is functionally equivalent,
but has defined behavior.
With this change, fate-idct8x8 compiled with --fsanitize=undefined works.
Bug-Id: 686
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
