proresdec: Properly make sure an index doesn't run past the limit

If idx equaled num_coeffs - 1 on entry to the loop, the previous
check failed to break out of the loop.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
