image/gif: avoid decoding past the first frame in decode()

The existing decode() method offers the ability to keep just one
frame of the GIF image, however it will read and decompress all
subsequent frames regardless.

Fixes #41142

src/image/gif/reader.go

@@ -250,6 +250,10 @@ func (d *decoder) decode(r io.Reader, configOnly, keepAllFrames bool) error {
 				return err
 			}
 
+			if !keepAllFrames && len(d.image) == 1 {
+				return nil
+			}
+
 		case sTrailer:
 			if len(d.image) == 0 {
 				return fmt.Errorf("gif: missing image data")

src/image/gif/reader_test.go

@@ -379,7 +379,7 @@ func TestLoopCount(t *testing.T) {
 
 func TestUnexpectedEOF(t *testing.T) {
 	for i := len(testGIF) - 1; i >= 0; i-- {
-		_, err := Decode(bytes.NewReader(testGIF[:i]))
+		_, err := DecodeAll(bytes.NewReader(testGIF[:i]))
 		if err == errNotEnough {
 			continue
 		}