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>

3 files changed, 32 insertions, 0 deletions

diff --git a/misc/cgo/errors/issue11097a.go b/misc/cgo/errors/issue11097a.go
new file mode 100644
index 0000000000..4508213cb4
--- /dev/null
+++ b/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
+}
diff --git a/misc/cgo/errors/issue11097b.go b/misc/cgo/errors/issue11097b.go
new file mode 100644
index 0000000000..68c5c7c64c
--- /dev/null
+++ b/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
+}
diff --git a/misc/cgo/errors/test.bash b/misc/cgo/errors/test.bash
index c880ad65c2..25ab249940 100755
--- a/misc/cgo/errors/test.bash
+++ b/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