image/jpeg: return io.ErrUnexpectedEOF on truncated data

Decoder calls fill from readFull, ignore and readByte and
readByte did not check returned io.EOF.

This change moves io.EOF translation inside fill.

name                 old speed      new speed      delta
DecodeBaseline-8     67.4MB/s ± 0%  67.3MB/s ± 0%  -0.20%  (p=0.000 n=16+19)
DecodeProgressive-8  43.7MB/s ± 0%  43.6MB/s ± 0%  -0.06%  (p=0.013 n=17+19)

Fixes #56724

src/image/jpeg/huffman.go

@@ -49,7 +49,7 @@ func (d *decoder) ensureNBits(n int32) error {
 	for {
 		c, err := d.readByteStuffedByte()
 		if err != nil {
-			if err == io.EOF {
+			if err == io.ErrUnexpectedEOF {
 				return errShortHuffmanData
 			}
 			return err

src/image/jpeg/reader.go

@@ -164,7 +164,10 @@ func (d *decoder) fill() error {
 	n, err := d.r.Read(d.bytes.buf[d.bytes.j:])
 	d.bytes.j += n
 	if n > 0 {
-		err = nil
+		return nil
+	}
+	if err == io.EOF {
+		err = io.ErrUnexpectedEOF
 	}
 	return err
 }
@@ -261,9 +264,6 @@ func (d *decoder) readFull(p []byte) error {
 			break
 		}
 		if err := d.fill(); err != nil {
-			if err == io.EOF {
-				err = io.ErrUnexpectedEOF
-			}
 			return err
 		}
 	}
@@ -291,9 +291,6 @@ func (d *decoder) ignore(n int) error {
 			break
 		}
 		if err := d.fill(); err != nil {
-			if err == io.EOF {
-				err = io.ErrUnexpectedEOF
-			}
 			return err
 		}
 	}

src/image/jpeg/reader_test.go

@@ -490,6 +490,20 @@ func TestExtraneousData(t *testing.T) {
 	}
 }
 
+func TestIssue56724(t *testing.T) {
+	b, err := os.ReadFile("../testdata/video-001.jpeg")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	b = b[:24] // truncate image data
+
+	_, err = Decode(bytes.NewReader(b))
+	if err != io.ErrUnexpectedEOF {
+		t.Errorf("got: %v, want: %v", err, io.ErrUnexpectedEOF)
+	}
+}
+
 func benchmarkDecode(b *testing.B, filename string) {
 	data, err := os.ReadFile(filename)
 	if err != nil {