go/doc: don't panic if method is missing recv type

Fixes #17788

Change-Id: I2f8a11321dc8f10bebbc8df90ba00ec65b9ee0fa
Reviewed-on: https://go-review.googlesource.com/32790Reviewed-by: Russ Cox <rsc@golang.org>

src/go/doc/reader.go

@@ -362,6 +362,11 @@ func (r *reader) readFunc(fun *ast.FuncDecl) {
 	// associate methods with the receiver type, if any
 	if fun.Recv != nil {
 		// method
+		if len(fun.Recv.List) == 0 {
+			// should not happen (incorrect AST); (See issue 17788)
+			// don't show this method
+			return
+		}
 		recvTypeName, imp := baseTypeName(fun.Recv.List[0].Type)
 		if imp {
 			// should not happen (incorrect AST);

src/go/doc/testdata/issue17788.0.golden

+// 
+PACKAGE issue17788
+
+IMPORTPATH
+	testdata/issue17788
+
+FILENAMES
+	testdata/issue17788.go

src/go/doc/testdata/issue17788.1.golden

+// 
+PACKAGE issue17788
+
+IMPORTPATH
+	testdata/issue17788
+
+FILENAMES
+	testdata/issue17788.go

src/go/doc/testdata/issue17788.2.golden

+// 
+PACKAGE issue17788
+
+IMPORTPATH
+	testdata/issue17788
+
+FILENAMES
+	testdata/issue17788.go

src/go/doc/testdata/issue17788.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.
+
+package issue17788
+
+func ( /* receiver type */ ) f0() {
+}