DNGDecoder: fix UB in DEFAULTSCALE handling
DNGDecoder: fix UB in DEFAULTSCALE handling

Avoid division by zero.
While there, i don't suppose they should be non-positive, either.

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