avcodec/diracdec: Fix Assertion frame->buf[0] failed at libavcodec/decode.c:610

Fixes: 1487/clusterfuzz-testcase-minimized-6288036495097856

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