encoding/xml: fix valid character range

Section 2.2 of the referenced spec http://www.xml.com/axml/testaxml.htm
defines 0xD7FF as a (sub)range boundary, not 0xDF77.

Fixes #25172

Change-Id: Ic5a3328cd46ef6474b8e93c4a343dcfba0e6511f
Reviewed-on: https://go-review.googlesource.com/109495
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/encoding/xml/xml_test.go

@@ -650,6 +650,20 @@ func TestDisallowedCharacters(t *testing.T) {
 	}
 }
 
+func TestIsInCharacterRange(t *testing.T) {
+	invalid := []rune{
+		utf8.MaxRune + 1,
+		0xD800, // surrogate min
+		0xDFFF, // surrogate max
+		-1,
+	}
+	for _, r := range invalid {
+		if isInCharacterRange(r) {
+			t.Errorf("rune %U considered valid", r)
+		}
+	}
+}
+
 var procInstTests = []struct {
 	input  string
 	expect [2]string