go/scanner: don't allow "0x" and "0X" as integers

R=gri
CC=golang-dev
https://golang.org/cl/4560047

src/pkg/go/scanner/scanner.go

@@ -297,6 +297,10 @@ func (S *Scanner) scanNumber(seenDecimalPoint bool) token.Token {
 			// hexadecimal int
 			S.next()
 			S.scanMantissa(16)
+			if S.offset-offs <= 2 {
+				// only scanned "0x" or "0X"
+				S.error(offs, "illegal hexadecimal number")
+			}
 		} else {
 			// octal int or float
 			seenDecimalDigit := false

src/pkg/go/scanner/scanner_test.go

@@ -672,6 +672,8 @@ var errors = []struct {
 	{"078e0", token.FLOAT, 0, ""},
 	{"078", token.INT, 0, "illegal octal number"},
 	{"07800000009", token.INT, 0, "illegal octal number"},
+	{"0x", token.INT, 0, "illegal hexadecimal number"},
+	{"0X", token.INT, 0, "illegal hexadecimal number"},
 	{"\"abc\x00def\"", token.STRING, 4, "illegal character NUL"},
 	{"\"abc\x80def\"", token.STRING, 4, "illegal UTF-8 encoding"},
 }