avcodec/dca_lbr: Fix off by 1 error in freq check

Fixes out of array read
Fixes: 510/clusterfuzz-testcase-5737865715646464

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