Implement rewrite logic for assigning a temporary scoped_refptr to T*.

tools/clang/rewrite_scoped_refptr/tests/test12-original.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <iterator>
 #include <map>
 #include <string>
 
 #include "scoped_refptr.h"
 
@@ skipping 22 matching lines @@
 };
 
 void TestsAScopedRefptr() {
   MyMap map;
   map["foo"] = new Foo;
   map["bar"] = new Foo;
   MyIter my_begin(map.begin());
   MyIter my_end(map.end());
   for (MyIter it = my_begin; it != my_end; ++it) {
     const Foo* item = NULL;
-    item = *it;
+    if (*it)
+      item = *it;
   }
 }