runtime: fix defer of nil func

Fixes #8047.

LGTM=r, iant
R=golang-codereviews, r, iant
CC=dvyukov, golang-codereviews, khr
https://golang.org/cl/105140044

src/pkg/runtime/stack.c

@@ -856,7 +856,12 @@ runtime·newstack(void)
 void
 runtime·gostartcallfn(Gobuf *gobuf, FuncVal *fv)
 {
-	runtime·gostartcall(gobuf, fv->fn, fv);
+	void *fn;
+
+	fn = nil;
+	if(fv != nil)
+		fn = fv->fn;
+	runtime·gostartcall(gobuf, fn, fv);
 }
 
 // Maybe shrink the stack being used by gp.

test/fixedbugs/issue8047b.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. Defer setup during panic shouldn't crash for nil defer.
+
+package main
+
+func main() {
+	defer func() {
+		recover()
+	}()
+	f()
+}
+
+func f() {
+	var g func()
+	defer g()
+	panic(1)
+}