by making gain unsigned we have 1 bit more available
alternatively we can clip twice as in the g729 reference

Fixes: left shift of 23404 by 17 places cannot be represented in type 'int'
Fixes: 61728/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ACELP_KELVIN_fuzzer-6280412547383296

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