avcodec/diracdec: Fix off by 1 error in quant check

Fixes: out of array read
Fixes: 1781/clusterfuzz-testcase-minimized-4617176877105152

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
