avcodec/aacdec_fixed: Fix runtime error: left shift of negative value -1

Fixes: 1535/clusterfuzz-testcase-minimized-5826695535788032

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>