avcodec/lagarith: Fix runtime error: left shift of 1 by 31 places cannot be represented in type 'int'

Fixes: 1764/clusterfuzz-testcase-minimized-5394243164045312

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