avcodec/diracdec: Fixes integer overflow

Fixes: runtime error: signed integer overflow: 340018243 * 27 cannot be represented in type 'int'
Fixes: 2861/clusterfuzz-testcase-minimized-5361070510178304

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