cmd/cgo: fix panic on references to non-existing C types

cgo panics in Package.rewriteRef for:

var a = C.enum_test(1)
or
p := new(C.enum_test)

when the corresponding enum type is not defined.

Check nil values for Type fields and issue a proper
error instead.

Fixes #11097
Updates #12160

Change-Id: I5821d29097ef0a36076ec5273125b09846c7d832
Reviewed-on: https://go-review.googlesource.com/15264
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/errors/issue11097a.go

@@ -0,0 +1,15 @@
+// Copyright 2015 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
+
+/*
+//enum test { foo, bar };
+*/
+import "C"
+
+func main() {
+	var a = C.enum_test(1) // ERROR HERE
+	_ = a
+}

misc/cgo/errors/issue11097b.go

@@ -0,0 +1,15 @@
+// Copyright 2015 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
+
+/*
+//enum test { foo, bar };
+*/
+import "C"
+
+func main() {
+	p := new(C.enum_test) // ERROR HERE
+	_ = p
+}

misc/cgo/errors/test.bash

@@ -31,6 +31,8 @@ check err2.go
 check err3.go
 check issue7757.go
 check issue8442.go
+check issue11097a.go
+check issue11097b.go
 
 rm -rf errs _obj
 exit 0

src/cmd/cgo/gcc.go

@@ -607,6 +607,10 @@ func (p *Package) rewriteRef(f *File) {
 			if r.Name.Kind != "func" {
 				if r.Name.Kind == "type" {
 					r.Context = "type"
+					if r.Name.Type == nil {
+						error_(r.Pos(), "invalid conversion to C.%s: undefined C type '%s'", fixGo(r.Name.Go), r.Name.C)
+						break
+					}
 					expr = r.Name.Type.Go
 					break
 				}
@@ -658,6 +662,10 @@ func (p *Package) rewriteRef(f *File) {
 				}
 			} else if r.Name.Kind == "type" {
 				// Okay - might be new(T)
+				if r.Name.Type == nil {
+					error_(r.Pos(), "expression C.%s: undefined C type '%s'", fixGo(r.Name.Go), r.Name.C)
+					break
+				}
 				expr = r.Name.Type.Go
 			} else if r.Name.Kind == "var" {
 				expr = &ast.StarExpr{Star: (*r.Expr).Pos(), X: expr}