image/gif: set default loop count to 0 when app ext. is not present

It was otherwise not being preserved across
specific Decode->Encode->Decode calls.

Fixes #11287

Change-Id: I40602da7fa39ec67403bed52ff403f361c6171bb
Reviewed-on: https://go-review.googlesource.com/11256Reviewed-by: Nigel Tao <nigeltao@golang.org>

src/image/gif/reader.go

@@ -271,7 +271,6 @@ func (d *decoder) readHeaderAndScreenDescriptor() error {
 		}
 	}
 	// d.tmp[12] is the Pixel Aspect Ratio, which is ignored.
-	d.loopCount = -1
 	return nil
 }
 

src/image/gif/reader_test.go

@@ -253,3 +253,24 @@ func TestPixelOutsidePaletteRange(t *testing.T) {
 		try(t, b.Bytes(), want)
 	}
 }
+
+func TestLoopCount(t *testing.T) {
+	data := []byte("GIF89a000\x00000,0\x00\x00\x00\n\x00" +
+		"\n\x00\x80000000\x02\b\xf01u\xb9\xfdal\x05\x00;")
+	img, err := DecodeAll(bytes.NewReader(data))
+	if err != nil {
+		t.Fatal("DecodeAll:", err)
+	}
+	w := new(bytes.Buffer)
+	err = EncodeAll(w, img)
+	if err != nil {
+		t.Fatal("EncodeAll:", err)
+	}
+	img1, err := DecodeAll(w)
+	if err != nil {
+		t.Fatal("DecodeAll:", err)
+	}
+	if img.LoopCount != img1.LoopCount {
+		t.Errorf("loop count mismatch: %d vs %d", img.LoopCount, img1.LoopCount)
+	}
+}