Don't resize inputs with zero width or height

Inputs having zero as both width and height were caught, but either one
alone caused a panic. This adds a test showing the old failing behavior
and adds a check for non-positive width/height. Since Resize doesn't
have an error return value, the behavior in this case is to return the
original image.

Fixes nfnt#52

resize.go

@@ -78,6 +78,7 @@ var blur = 1.0
 // If one of the parameters width or height is set to 0, its size will be calculated so that
 // the aspect ratio is that of the originating image.
 // The resizing algorithm uses channels for parallel computation.
+// If the input image has width or height of 0, it is returned unchanged.
 func Resize(width, height uint, img image.Image, interp InterpolationFunction) image.Image {
 	scaleX, scaleY := calcFactors(width, height, float64(img.Bounds().Dx()), float64(img.Bounds().Dy()))
 	if width == 0 {
@@ -92,6 +93,11 @@ func Resize(width, height uint, img image.Image, interp InterpolationFunction) i
 		return img
 	}
 
+	// Input image has no pixels
+	if img.Bounds().Dx() <= 0 || img.Bounds().Dy() <= 0 {
+		return img
+	}
+
 	if interp == NearestNeighbor {
 		return resizeNearest(width, height, scaleX, scaleY, img, interp)
 	}

resize_test.go

@@ -37,6 +37,20 @@ func Test_ZeroImg(t *testing.T) {
 	}
 }
 
+func Test_HalfZeroImg(t *testing.T) {
+	zeroImg := image.NewGray16(image.Rect(0, 0, 0, 100))
+
+	m := Resize(0, 1, zeroImg, NearestNeighbor)
+	if m.Bounds() != zeroImg.Bounds() {
+		t.Fail()
+	}
+
+	m = Resize(1, 0, zeroImg, NearestNeighbor)
+	if m.Bounds() != zeroImg.Bounds() {
+		t.Fail()
+	}
+}
+
 func Test_CorrectResize(t *testing.T) {
 	zeroImg := image.NewGray16(image.Rect(0, 0, 256, 256))