Pass a valid new PixelWand pointer instead of a null pointer to avoid crash

The specific GetImagePixelColor call reference in issue #11, along with a few other calls, were passing a null PixelWand pointer to the C call. This fix creates a new valid PixelWand and passes it. The caller would be responsible for destroying it. 

Updated test script from original poster:

```
package main

import "github.com/gographics/imagick/imagick"

func main() {

	imagick.Initialize()
	defer imagick.Terminate()

	mw := imagick.NewMagickWand()
	defer mw.Destroy()

	pw := imagick.NewPixelWand()
	defer pw.Destroy()
	pw.SetColor("red")

	mw.NewImage(50, 50, pw)
	pw.SetColor("black")
	mw.NewImage(50, 50, pw)
	pw.SetColor("blue")
	mw.NewImage(50, 50, pw)

	mw.ResetIterator()

	combine := mw.AppendImages(true)

	combine.WriteImage("test.png")

	target, _ := combine.GetImagePixelColor(1, 1)
	defer target.Destroy()

	target, _ = combine.GetImageBackgroundColor()
	defer target.Destroy()

	target, _ = combine.GetImageBorderColor()
	defer target.Destroy()

	target, _ = combine.GetImageMatteColor()
	defer target.Destroy()

	target, _ = combine.GetImageColormapColor(0)
	defer target.Destroy()

	pw.SetColor("blue")
	combine.FloodfillPaintImage(imagick.CHANNELS_DEFAULT, pw, 1, target, 1, 1, false)
	combine.WriteImage("test22.png")

}

```