fix: bug in SetPages (#421)

Co-authored-by: qinghuan <[email protected]>

Author: zhifengzhuang

vips/image.go

@@ -766,7 +766,7 @@ func (r *ImageRef) SetPages(pages int) error {
 		return err
 	}
 
-	vipsSetImageNPages(r.image, pages)
+	vipsSetImageNPages(out, pages)
 
 	r.setImage(out)
 	return nil

vips/image_test.go

@@ -1169,6 +1169,18 @@ func TestImageRef_HistogramEntropy(t *testing.T) {
 	require.True(t, e > 0)
 }
 
+func TestImageRef_SetPages(t *testing.T) {
+	Startup(nil)
+
+	image, err := NewImageFromFile(resources + "gif-animated.gif")
+	require.NoError(t, err)
+	require.Equal(t, 8, image.Pages())
+
+	err = image.SetPages(3)
+	require.NoError(t, err)
+	require.Equal(t, 3, image.Pages())
+}
+
 // TODO unit tests to cover:
 // NewImageFromReader failing test
 // NewImageFromFile failing test