syscall: allow empty string argument to SetsockoptString

Don't panic with "index out of range" on empty string argument.

Fixes golang/go#31277

Change-Id: I005f9523caec76337cb2ec87272a6be4736bce18
Reviewed-on: https://go-review.googlesource.com/c/go/+/170937
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/syscall_unix.go

@@ -323,7 +323,11 @@ func SetsockoptLinger(fd, level, opt int, l *Linger) (err error) {
 }
 
 func SetsockoptString(fd, level, opt int, s string) (err error) {
-	return setsockopt(fd, level, opt, unsafe.Pointer(&[]byte(s)[0]), uintptr(len(s)))
+	var p unsafe.Pointer
+	if len(s) > 0 {
+		p = unsafe.Pointer(&[]byte(s)[0])
+	}
+	return setsockopt(fd, level, opt, p, uintptr(len(s)))
 }
 
 func SetsockoptTimeval(fd, level, opt int, tv *Timeval) (err error) {

src/syscall/syscall_unix_test.go

@@ -355,3 +355,11 @@ func TestSeekFailure(t *testing.T) {
 		t.Fatalf("Seek(-1, 0, 0) return error with empty message")
 	}
 }
+
+func TestSetsockoptString(t *testing.T) {
+	// should not panic on empty string, see issue #31277
+	err := syscall.SetsockoptString(-1, 0, 0, "")
+	if err == nil {
+		t.Fatalf("SetsockoptString: did not fail")
+	}
+}