cmd/gc: remove bad check for BADWIDTH

This check for BADWIDTH might happen while in defercheckwidth, making it raise errors for non-erroneous situations.

Fixes #4495.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6927043

src/cmd/gc/typecheck.c

@@ -1791,8 +1791,6 @@ looktypedot(Node *n, Type *t, int dostrcmp)
 		if(f1 == T)
 			return 0;
 
-		if(f1->width == BADWIDTH)
-			fatal("lookdot badwidth %T %p", f1, f1);
 		n->right = methodname(n->right, t);
 		n->xoffset = f1->width;
 		n->type = f1->type;

test/fixedbugs/issue4495.go

+// run
+
+// 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.
+
+package main
+
+type I interface {
+	m() int
+}
+
+type T struct{}
+
+func (T) m() int {
+	return 3
+}
+
+var t T
+
+var ret = I.m(t)
+
+func main() {
+	if ret != 3 {
+		println("ret = ", ret)
+		panic("ret != 3")
+	}
+}
+