avcodec/mlpdsp: Fix undefined shift ff_mlp_pack_output()

Fixes: runtime error: left shift of negative value -7862264
Fixes: 4074/clusterfuzz-testcase-minimized-4516104123711488

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