avcodec/aacdec_fixed: Check s for being too small

Fixes: runtime error: shift exponent -8 is negative
Fixes: 2286/clusterfuzz-testcase-minimized-5711764169687040

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