Fixes: shift exponent 32 is too large for 32-bit type 'int'
Fixes: 21200/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XAN_DPCM_fuzzer-5754704894361600

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>