gc: don't fault on return outside function

Fixes #2598.

R=golang-dev, ality, minux.ma, mpimenov, rsc
CC=golang-dev
https://golang.org/cl/5510043

src/cmd/gc/go.y

@@ -1618,7 +1618,7 @@ non_dcl_stmt:
 	{
 		$$ = nod(ORETURN, N, N);
 		$$->list = $2;
-		if($$->list == nil) {
+		if($$->list == nil && curfn != N) {
 			NodeList *l;
 			
 			for(l=curfn->dcl; l; l=l->next) {

test/fixedbugs/bug394.go

+// errchk $G $D/$F.go
+
+// Copyright 2011 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 2598
+package foo
+
+return nil // ERROR "non-declaration statement outside function body"