Fixes: left shift of negative value -1
Fixes: 18859/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ADPCM_XA_fuzzer-5748474213040128

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