avcodec/aacsbr_fixed: Fix division by zero in sbr_gain_calc()

Fixes: 3642/clusterfuzz-testcase-minimized-5443853801750528

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