avcodec/flacdsp: Fix: runtime error: signed integer overflow: -1027555328 + -1226681270 cannot be represented in type 'int'

Fixes: 673/clusterfuzz-testcase-5948736536576000

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