avcodec/aacsbr_fixed: Fix multiple runtime error: left shift of negative value -407

Fixes: 1674/clusterfuzz-testcase-minimized-6092531563495424
Fixes: 1686/clusterfuzz-testcase-minimized-6282691643179008

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