DngDecoder: if ASSHOTNEUTRAL mul is zero, avoid div-by-zero
DngDecoder: if ASSHOTNEUTRAL mul is zero, avoid div-by-zero

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