fmt: don't unread eof scanning %x

When scanning a hex byte at EOF, the code was ungetting the eof,
which backed up the input and caused double-scanning of a byte.

Delete the call to UnreadRune.

This line appeared in 1.5 for some reason; it was not in 1.4 and
should be removed again for 1.5

Fixes #12090.

Change-Id: Iad1ce8e7db8ec26615c5271310f4b0228cca7d78
Reviewed-on: https://go-review.googlesource.com/13461Reviewed-by: Andrew Gerrand <adg@golang.org>

src/fmt/scan.go

@@ -888,7 +888,6 @@ func hexDigit(d rune) (int, bool) {
 func (s *ss) hexByte() (b byte, ok bool) {
 	rune1 := s.getRune()
 	if rune1 == eof {
-		s.UnreadRune()
 		return
 	}
 	value1, ok := hexDigit(rune1)

src/fmt/scan_test.go

@@ -1128,3 +1128,31 @@ func TestScanfNewlineMatchFormat(t *testing.T) {
 		}
 	}
 }
+
+// Test for issue 12090: Was unreading at EOF, double-scanning a byte.
+
+type hexBytes [2]byte
+
+func (h *hexBytes) Scan(ss ScanState, verb rune) error {
+	var b []byte
+	_, err := Fscanf(ss, "%4x", &b)
+	if err != nil {
+		panic(err) // Really shouldn't happen.
+	}
+	copy((*h)[:], b)
+	return err
+}
+
+func TestHexByte(t *testing.T) {
+	var h hexBytes
+	n, err := Sscanln("0123\n", &h)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 1 {
+		t.Fatalf("expected 1 item; scanned %d", n)
+	}
+	if h[0] != 0x01 || h[1] != 0x23 {
+		t.Fatalf("expected 0123 got %x", h)
+	}
+}