cmd/vet: don't crash in cgo checker if type is unknown

Fixes #14201.

Change-Id: Ib61f8c00cae72463f59b90ae199fbdc1e7422a79
Reviewed-on: https://go-review.googlesource.com/19174
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/vet/cgo.go

@@ -72,6 +72,9 @@ func cgoBaseType(f *File, arg ast.Expr) types.Type {
 		}
 		// Here arg is *f(v).
 		t := f.pkg.types[call.Fun].Type
+		if t == nil {
+			break
+		}
 		ptr, ok := t.Underlying().(*types.Pointer)
 		if !ok {
 			break

src/cmd/vet/testdata/cgo2.go

+// Copyright 2016 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.
+
+// Test the cgo checker on a file that doesn't use cgo.
+
+package testdata
+
+var _ = C.f(*p(**p))