Fixes: negative shift
Fixes: 22499/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ADPCM_XA_fuzzer-5765452130418688

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