avcodec/aacdec_fixed: fix invalid shift in predict()

Fixes: runtime error: shift exponent -2 is negative
Fixes: 2818/clusterfuzz-testcase-minimized-5062943676825600

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