avcodec/aacdec_fixed: Fix multiple shift exponent 33 is too large for 32-bit type 'int'

Fixes: 1471/clusterfuzz-testcase-minimized-6376460543590400

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