go/scanner: don't accept '\x0g' character escape (bug fix)

Added more test cases.

R=rsc
CC=golang-dev
https://golang.org/cl/2804041

src/pkg/go/scanner/scanner.go

@@ -368,15 +368,19 @@ func (S *Scanner) scanEscape(quote int) {
 	}
 
 	var x uint32
-	for ; i > 0; i-- {
+	for ; i > 0 && S.ch != quote && S.ch >= 0; i-- {
 		d := uint32(digitVal(S.ch))
-		if d > base {
+		if d >= base {
 			S.error(S.pos, "illegal character in escape sequence")
-			return
+			break
 		}
 		x = x*base + d
 		S.next()
 	}
+	// in case of an error, consume remaining chars
+	for ; i > 0 && S.ch != quote && S.ch >= 0; i-- {
+		S.next()
+	}
 	if x > max || 0xd800 <= x && x < 0xe000 {
 		S.error(pos, "escape sequence is invalid Unicode code point")
 	}

src/pkg/go/scanner/scanner_test.go

@@ -610,8 +610,18 @@ var errors = []struct {
 	pos int
 	err string
 }{
-	{"\"\"", token.STRING, 0, ""},
-	{"\"", token.STRING, 0, "string not terminated"},
+	{`#`, token.ILLEGAL, 0, "illegal character '#' (U+23)"},
+	{`' '`, token.CHAR, 0, ""},
+	{`''`, token.CHAR, 0, "illegal character literal"},
+	{`'\8'`, token.CHAR, 2, "unknown escape sequence"},
+	{`'\08'`, token.CHAR, 3, "illegal character in escape sequence"},
+	{`'\x0g'`, token.CHAR, 4, "illegal character in escape sequence"},
+	{`'\Uffffffff'`, token.CHAR, 2, "escape sequence is invalid Unicode code point"},
+	{`'`, token.CHAR, 0, "character literal not terminated"},
+	{`""`, token.STRING, 0, ""},
+	{`"`, token.STRING, 0, "string not terminated"},
+	{"``", token.STRING, 0, ""},
+	{"`", token.STRING, 0, "string not terminated"},
 	{"/**/", token.COMMENT, 0, ""},
 	{"/*", token.COMMENT, 0, "comment not terminated"},
 	{"//\n", token.COMMENT, 0, ""},