gdImageScaleTwoPass memory leak fix

Fixing memory leak in gdImageScaleTwoPass, as reported by @cmb69 and
confirmed by @vapier.  This bug actually bit me in production and I'm
very thankful that it was reported with an easy fix.

Fixes #173.
static double filter_catmullrom(const double x)
{
	if (x < -2.0)
		return(0.0f);
	if (x < -1.0)
		return(0.5f*(4.0f+x*(8.0f+x*(5.0f+x))));
	if (x < 0.0)
		return(0.5f*(2.0f+x*x*(-5.0f-3.0f*x)));
	if (x < 1.0)
		return(0.5f*(2.0f+x*x*(-5.0f+3.0f*x)));
	if (x < 2.0)
		return(0.5f*(4.0f+x*(-8.0f+x*(5.0f-x))));
	return(0.0f);
}

static double filter_catmullrom(const double x)
{
	if (x < -2.0)
		return(0.0f);
	if (x < -1.0)
		return(0.5f*(4.0f+x*(8.0f+x*(5.0f+x))));
	if (x < 0.0)
		return(0.5f*(2.0f+x*x*(-5.0f-3.0f*x)));
	if (x < 1.0)
		return(0.5f*(2.0f+x*x*(-5.0f+3.0f*x)));
	if (x < 2.0)
		return(0.5f*(4.0f+x*(-8.0f+x*(5.0f-x))));
	return(0.0f);
}
