DngDecoder::decodeData(): slices: assert that the slice is within image.
DngDecoder::decodeData(): slices: assert that the slice is within image.

offY is the row where the slice starts. if it is out of the image,
then that is an extra (invalid?) slice.
Catch it here until it crashes something later on.

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