avcodec/g723_1dec: Fix runtime error: left shift of negative value -1

Fixes: 1504/clusterfuzz-testcase-minimized-6249212138225664

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