fix uninitialized variable warning for vs2012 compilation

net/spdy/spdy_header_block.cc

 // Copyright (c) 2012 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 "net/spdy/spdy_header_block.h"
 
 #include "base/values.h"
 
 namespace net {
 
@@ skipping 10 matching lines @@
   dict->Set("headers", headers_dict);
   return dict;
 }
 
 bool SpdyHeaderBlockFromNetLogParam(
     const base::Value* event_param,
     SpdyHeaderBlock* headers) {
   headers->clear();
 
   const base::DictionaryValue* dict;
-  const base::DictionaryValue* header_dict;
+  const base::DictionaryValue* header_dict = NULL;
 
   if (!event_param ||
       !event_param->GetAsDictionary(&dict) ||
       !dict->GetDictionary("headers", &header_dict)) {
     return false;
   }
 
   for (base::DictionaryValue::key_iterator it = header_dict->begin_keys();
        it != header_dict->end_keys();
        ++it) {
     std::string value;
     if (!header_dict->GetString(*it, &(*headers)[*it])) {
       headers->clear();
       return false;
     }
   }
   return true;
 }
 
 }  // namespace net