avcodec/lpc: signed integer overflow in compute_lpc_coefs() (aacdec_fixed)

Fixes: runtime error: signed integer overflow: -1575818955 + -915383657 cannot be represented in type 'int'
Fixes: 2224/clusterfuzz-testcase-minimized-6208559949807616

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