avcodec/g723_1: Fix runtime error: signed integer overflow: -1013481472 + -1139123755 cannot be represented in type 'int'

See: LsptoA() and L_add()
Fixes: 1758/clusterfuzz-testcase-minimized-6054857184116736

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