test: add test for issue 17039

Change-Id: Ieb3d605f03a7185a707621bef7160090c9bdb51f
Reviewed-on: https://go-review.googlesource.com/28873Reviewed-by: Alan Donovan <adonovan@google.com>

test/fixedbugs/issue17039.go

+// run
+
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+type S []S
+
+func main() {
+	var s S
+	s = append(s, s) // append a nil value to s
+	if s[0] != nil {
+		println("BUG: s[0] != nil")
+	}
+}