runtime: add test for issue 8047.

Make sure stack copier doesn't barf on a nil defer.
Bug was fixed in https://golang.org/cl/101800043
This change just adds a test.

Fixes #8047

LGTM=dvyukov, rsc
R=dvyukov, rsc
CC=golang-codereviews
https://golang.org/cl/108840043

test/fixedbugs/issue8047.go

+// run
+
+// Copyright 2014 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.
+
+// Issue 8047.  Stack copier shouldn't crash if there
+// is a nil defer.
+
+package main
+
+func stackit(n int) {
+	if n == 0 {
+		return
+	}
+	stackit(n - 1)
+}
+
+func main() {
+	defer func() {
+		// catch & ignore panic from nil defer below
+		err := recover()
+		if err == nil {
+			panic("defer of nil func didn't panic")
+		}
+	}()
+	defer ((func())(nil))()
+	stackit(1000)
+}