Fixes: runtime error: signed integer overflow: 2140143616 + 254665816 cannot be represented in type 'int'
Fixes: 45982/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ADPCM_XMD_fuzzer-6690181676924928

As a sideeffect this simplifies the equation, the high bits are different after this but only
the low 16bits are stored and used in later steps.

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>