avcodec/aacdec_fixed: Fix runtime error: shift exponent 34 is too large for 32-bit type 'int'

Fixes: 1721/clusterfuzz-testcase-minimized-4719352135811072

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


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