avcodec/diracdec: Check weight_log2denom

Fixes: runtime error: shift exponent -1 is negative
Fixes: 2742/clusterfuzz-testcase-minimized-5724322402402304
Fixes: 2744/clusterfuzz-testcase-minimized-4672435653705728
Fixes: 2749/clusterfuzz-testcase-minimized-5298741273690112

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