Fix AreaWidth calculation

[fisherking]

Fixes #249

[Dynom]

Excellent, thanks!

I would like to see a test-case to prove the fix. We're slightly behind on coverage and this is an excellent candidate to improve it for that function. I've quickly wrote one you could include in resizer_test.go. It requires a small change to resizer.go as well, I've included a patch you could apply to make it all work.

@h2non Something we want to merge back soon I suppose :-)

diff --git a/resizer.go b/resizer.go
index 98a95ae..934b937 100644
--- a/resizer.go
+++ b/resizer.go
@@ -12,6 +12,10 @@ import (
 	"math"
 )

+var (
+	ErrExtractAreaParamsRequired = errors.New("extract area width/height params are required")
+)
+
 // resizer is used to transform a given image as byte buffer
 // with the passed options.
 func resizer(buf []byte, o Options) ([]byte, error) {
@@ -279,7 +283,7 @@ func extractOrEmbedImage(image *C.VipsImage, o Options) (*C.VipsImage, error) {
 			o.AreaHeight = o.Height
 		}
 		if o.AreaWidth == 0 || o.AreaHeight == 0 {
-			return nil, errors.New("Extract area width/height params are required")
+			return nil, ErrExtractAreaParamsRequired
 		}
 		image, err = vipsExtract(image, o.Left, o.Top, o.AreaWidth, o.AreaHeight)
 		break
diff --git a/resizer_test.go b/resizer_test.go
index a4b54b0..6a116a2 100644
--- a/resizer_test.go
+++ b/resizer_test.go
@@ -364,6 +364,54 @@ func TestExtractCustomAxis(t *testing.T) {
 	Write("testdata/test_extract_custom_axis_out.jpg", newImg)
 }

+func TestExtractOrEmbedImage(t *testing.T) {
+	buf, _ := Read("testdata/test.jpg")
+	input, _, err := loadImage(buf)
+	if err != nil {
+		t.Fatalf("Unable to load image %s", err)
+	}
+
+	o := Options{
+		Top:    10,
+		Left:   10,
+		Width:  100,
+		Height: 200,
+
+		// Fields to test
+		AreaHeight: 0,
+		AreaWidth:  0,
+
+		Quality: 100, /* Needs a value to satisfy libvips */
+	}
+
+	result, err := extractOrEmbedImage(input, o)
+	if err != nil {
+		if err == ErrExtractAreaParamsRequired {
+			t.Fatalf("Expecting AreaWidth and AreaHeight to have been defined")
+		}
+
+		t.Fatalf("Unknown error occurred %s", err)
+	}
+
+	image, err := saveImage(result, o)
+	if err != nil {
+		t.Fatalf("Failed saving image %s", err)
+	}
+
+	test, err := Size(image)
+	if err != nil {
+		t.Fatalf("Failed fetching the size %s", err)
+	}
+
+	if test.Height != o.Height {
+		t.Errorf("Extract failed, resulting Height %d doesn't match %d", test.Height, o.Height)
+	}
+
+	if test.Width != o.Width {
+		t.Errorf("Extract failed, resulting Width %d doesn't match %d", test.Width, o.Width)
+	}
+}
+
 func TestConvert(t *testing.T) {
 	width, height := 300, 240
 	formats := [3]ImageType{PNG, WEBP, JPEG}