image/draw: fix crash in clip

Fixes #9177

Change-Id: I1c7e57f0f0a9b00fb3ddc7fa4844ac53ea6df46f
Reviewed-on: https://go-review.googlesource.com/1876Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/image/draw/clip_test.go

@@ -191,3 +191,13 @@ func TestClip(t *testing.T) {
 		}
 	}
 }
+
+func TestClipWithNilMP(t *testing.T) {
+	src := image.NewRGBA(image.Rect(0, 0, 100, 100))
+	// dst must be smaller than src for clipping to occur
+	dst := image.NewRGBA(image.Rect(50, 50, 100, 100))
+	r := image.Rect(0, 0, 100, 100)
+	sp := image.ZP
+	// issue 9177: floydSteinberg.Draw passes nil for mp, which used to cause clip to panic
+	clip(dst, &r, src, &sp, nil, nil)
+}

src/image/draw/draw.go

@@ -83,8 +83,10 @@ func clip(dst Image, r *image.Rectangle, src image.Image, sp *image.Point, mask
 	}
 	(*sp).X += dx
 	(*sp).Y += dy
-	(*mp).X += dx
-	(*mp).Y += dy
+	if mp != nil {
+		(*mp).X += dx
+		(*mp).Y += dy
+	}
 }
 
 func processBackward(dst Image, r image.Rectangle, src image.Image, sp image.Point) bool {