Double-check that the input image's density is valid

Fixes a bug that could result in division by zero, at least for a JPEG
source image.
Fixes issues #19, #20
IW_IMPL(void) iw_make_srgb_csdescr_2(struct iw_csdescr *cs)
{
	cs->cstype = IW_CSTYPE_SRGB;
	cs->gamma = 0.0;
}

IW_IMPL(void) iw_make_srgb_csdescr_2(struct iw_csdescr *cs)
{
	cs->cstype = IW_CSTYPE_SRGB;
	cs->gamma = 0.0;
}
