avcodec/aacsbr: Fix libavcodec/aacsbr.c:257:59: runtime error: division by zero

Fixes: 1882/clusterfuzz-testcase-minimized-5539735650959360

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