DngDecoder::decodeData(): strips: properly sanitize strip count.
DngDecoder::decodeData(): strips: properly sanitize strip count.

We may have not enough strips. We may have extra strips.
Also, muliplication may overflow. So use properly-rounding division.

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