avcodec/aacdec_fixed: Fix integer overflow in predict()

Fixes: runtime error: signed integer overflow: -2110708110 + -82837504 cannot be represented in type 'int'
Fixes: 3547/clusterfuzz-testcase-minimized-6009386439802880

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