avcodec/alacdsp: Make intermediates unsigned

Fixes: signed integer overflow: -14914387 + -2147418648 cannot be represented in type 'int'
Fixes: 46464/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALAC_fuzzer-474307197311385

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