MosDecoder::DecodePhaseOneC(): properly subtract.
MosDecoder::DecodePhaseOneC(): properly subtract.

Last UB in current sample set.

darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed