avcodec/aacdec_fixed: Fix runtime error: left shift of negative value -1297616

Fixes: 2195/clusterfuzz-testcase-minimized-4736721533009920

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