avcodec/ivi_dsp: Fix runtime error: left shift of negative value -2

Fixes: 1839/clusterfuzz-testcase-minimized-6238490993885184

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