unix: allow empty string argument to SetsockoptString

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

Follows CL 170937 which did the sane for package syscall.

Updates golang/go#31277

Change-Id: I4feb796d0d58d3637428ae69997cfa3ec28a6b01
Reviewed-on: https://go-review.googlesource.com/c/sys/+/170957
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_unix_test.go

@@ -403,6 +403,14 @@ func TestSeekFailure(t *testing.T) {
 	}
 }
 
+func TestSetsockoptString(t *testing.T) {
+	// should not panic on empty string, see issue #31277
+	err := unix.SetsockoptString(-1, 0, 0, "")
+	if err == nil {
+		t.Fatalf("SetsockoptString: did not fail")
+	}
+}
+
 func TestDup(t *testing.T) {
 	file, err := ioutil.TempFile("", "TestDup")
 	if err != nil {