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_mitchell(const double x)
{
#define KM_B (1.0f/3.0f)
#define KM_C (1.0f/3.0f)
#define KM_P0 ((  6.0f - 2.0f * KM_B ) / 6.0f)
#define KM_P2 ((-18.0f + 12.0f * KM_B + 6.0f * KM_C) / 6.0f)
#define KM_P3 (( 12.0f - 9.0f  * KM_B - 6.0f * KM_C) / 6.0f)
#define KM_Q0 ((  8.0f * KM_B + 24.0f * KM_C) / 6.0f)
#define KM_Q1 ((-12.0f * KM_B - 48.0f * KM_C) / 6.0f)
#define KM_Q2 ((  6.0f * KM_B + 30.0f * KM_C) / 6.0f)
#define KM_Q3 (( -1.0f * KM_B -  6.0f * KM_C) / 6.0f)

	if (x < -2.0)
		return(0.0f);
	if (x < -1.0)
		return(KM_Q0-x*(KM_Q1-x*(KM_Q2-x*KM_Q3)));
	if (x < 0.0f)
		return(KM_P0+x*x*(KM_P2-x*KM_P3));
	if (x < 1.0f)
		return(KM_P0+x*x*(KM_P2+x*KM_P3));
	if (x < 2.0f)
		return(KM_Q0+x*(KM_Q1+x*(KM_Q2+x*KM_Q3)));
	return(0.0f);
}

static double filter_mitchell(const double x)
{
#define KM_B (1.0f/3.0f)
#define KM_C (1.0f/3.0f)
#define KM_P0 ((  6.0f - 2.0f * KM_B ) / 6.0f)
#define KM_P2 ((-18.0f + 12.0f * KM_B + 6.0f * KM_C) / 6.0f)
#define KM_P3 (( 12.0f - 9.0f  * KM_B - 6.0f * KM_C) / 6.0f)
#define KM_Q0 ((  8.0f * KM_B + 24.0f * KM_C) / 6.0f)
#define KM_Q1 ((-12.0f * KM_B - 48.0f * KM_C) / 6.0f)
#define KM_Q2 ((  6.0f * KM_B + 30.0f * KM_C) / 6.0f)
#define KM_Q3 (( -1.0f * KM_B -  6.0f * KM_C) / 6.0f)

	if (x < -2.0)
		return(0.0f);
	if (x < -1.0)
		return(KM_Q0-x*(KM_Q1-x*(KM_Q2-x*KM_Q3)));
	if (x < 0.0f)
		return(KM_P0+x*x*(KM_P2-x*KM_P3));
	if (x < 1.0f)
		return(KM_P0+x*x*(KM_P2+x*KM_P3));
	if (x < 2.0f)
		return(KM_Q0+x*(KM_Q1+x*(KM_Q2+x*KM_Q3)));
	return(0.0f);
}
