cmd/compile: fix segfault in race instrumentation

Fixes #13265.

Change-Id: I792eb4ee26bef8a56e279e23f9802cb39019e0d0
Reviewed-on: https://go-review.googlesource.com/34929Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/walk.go

@@ -2187,6 +2187,9 @@ func callnew(t *types.Type) *Node {
 }
 
 func iscallret(n *Node) bool {
+	if n == nil {
+		return false
+	}
 	n = outervalue(n)
 	return n.Op == OINDREGSP
 }

test/fixedbugs/issue13265.go

+// errorcheck -0 -race
+
+// Copyright 2017 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 13265: nil pointer deref.
+
+package p
+
+func f() {
+    var c chan chan chan int
+    for ; ; <-<-<-c {
+    }
+}