avcodec/mlpdsp: Fix signed integer overflow, 2nd try

The outputted bits should match what is used in the lossless check

Fixes: runtime error: signed integer overflow: -538697856 * 256 cannot be represented in type 'int'
Fixes: 4326/clusterfuzz-testcase-minimized-5689449645080576

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