avcodec/amrwbdec: Fix division by 0 in find_hb_gain()

This restructures the code slightly toward D_UTIL_dec_synthesis()

Fixes: 7420/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AMRWB_fuzzer-6577305112543232

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>