AU: HttpFetcher: Fix segfault with multi fetcher and small transfers

http_fetcher.cc

 // Copyright (c) 2009 The Chromium OS 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 "update_engine/http_fetcher.h"
 
 using google::protobuf::Closure;
 using std::deque;
 using std::string;
 
@@ skipping 13 matching lines @@
   post_data_.insert(post_data_.end(), char_data, char_data + size);
 }
 
 // Proxy methods to set the proxies, then to pop them off.
 bool HttpFetcher::ResolveProxiesForUrl(const string& url, Closure* callback) {
   if (!proxy_resolver_) {
     LOG(INFO) << "Not resolving proxies (no proxy resolver).";
     no_resolver_idle_id_ = g_idle_add(utils::GlibRunClosure, callback);
     return true;
   }
+  CHECK_EQ(reinterpret_cast<Closure*>(NULL), callback_);
   callback_ = callback;
   return proxy_resolver_->GetProxiesForUrl(url,
                                            &HttpFetcher::StaticProxiesResolved,
                                            this);
 }
 
 void HttpFetcher::ProxiesResolved(const std::deque<std::string>& proxies) {
   no_resolver_idle_id_ = 0;
   if (!proxies.empty())
     SetProxies(proxies);
-  callback_->Run();
+  CHECK_NE(reinterpret_cast<Closure*>(NULL), callback_);
+  Closure* callback = callback_;
   callback_ = NULL;
+  // This may indirectly call back into ResolveProxiesForUrl():
+  callback->Run();
 }
 
 }  // namespace chromeos_update_engine