cmd/gc: fix internal compiler error with broken structs.

Fixes #4359.

R=golang-dev, daniel.morsing, rsc
CC=golang-dev
https://golang.org/cl/6834043

src/cmd/gc/typecheck.c

@@ -443,7 +443,7 @@ reswitch:
 		ok |= Etype;
 		n->op = OTYPE;
 		n->type = tostruct(n->list);
-		if(n->type == T)
+		if(n->type == T || n->type->broke)
 			goto error;
 		n->list = nil;
 		break;

test/fixedbugs/issue4359.go

+// errorcheck
+
+// Copyright 2012 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 4359: wrong handling of broken struct fields
+// causes "internal compiler error: lookdot badwidth".
+
+package main
+
+type T struct {
+	x T1 // ERROR "undefined"
+}
+
+func f() {
+	var t *T
+	_ = t.x
+}