avcodec/cngdec: Fix integer clipping

Fixes: runtime error: value -36211.7 is outside the range of representable values of type 'short'
Fixes: 2992/clusterfuzz-testcase-6649611793989632

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