strconv: CanBackquote should reject \x7F

It's a control character.
Fixes #7565.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/77300043

src/pkg/strconv/quote.go

@@ -144,7 +144,8 @@ func AppendQuoteRuneToASCII(dst []byte, r rune) []byte {
 // characters other than space and tab.
 func CanBackquote(s string) bool {
 	for i := 0; i < len(s); i++ {
-		if (s[i] < ' ' && s[i] != '\t') || s[i] == '`' {
+		c := s[i]
+		if (c < ' ' && c != '\t') || c == '`' || c == '\u007F' {
 			return false
 		}
 	}

src/pkg/strconv/quote_test.go

@@ -140,6 +140,7 @@ var canbackquotetests = []canBackquoteTest{
 	{string(29), false},
 	{string(30), false},
 	{string(31), false},
+	{string(0x7F), false},
 	{`' !"#$%&'()*+,-./:;<=>?@[\]^_{|}~`, true},
 	{`0123456789`, true},
 	{`ABCDEFGHIJKLMNOPQRSTUVWXYZ`, true},