strconv: check for empty string in UnquoteChar

The existing implementation panics on malformed input of an empty
string. strconv.Unquote validates the length of the inputs, but calling
UnquoteChar directly with an empty string leads to a panic, so add a
check for length. Also, add a test to go/constant to ensure that
MakeFromLiteral does not panic on malformed input such as
"const x = ''".

Change-Id: I4217e38db48a09a21ec414bbfb3087709da62904
Reviewed-on: https://go-review.googlesource.com/116215
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/constant/value_test.go

@@ -431,6 +431,7 @@ func TestUnknown(t *testing.T) {
 		MakeBool(false), // token.ADD ok below, operation is never considered
 		MakeString(""),
 		MakeInt64(1),
+		MakeFromLiteral("''", token.CHAR, 0),
 		MakeFromLiteral("-1234567890123456789012345678901234567890", token.INT, 0),
 		MakeFloat64(1.2),
 		MakeImag(MakeFloat64(1.2)),

src/strconv/quote.go

@@ -237,6 +237,10 @@ func unhex(b byte) (v rune, ok bool) {
 // If set to zero, it does not permit either escape and allows both quote characters to appear unescaped.
 func UnquoteChar(s string, quote byte) (value rune, multibyte bool, tail string, err error) {
 	// easy cases
+	if len(s) == 0 {
+		err = ErrSyntax
+		return
+	}
 	switch c := s[0]; {
 	case c == quote && (quote == '\'' || quote == '"'):
 		err = ErrSyntax