HostResolver: don't interpret NULL callback argument as a request to do synchronous resolution.

net/base/single_request_host_resolver.cc

 // Copyright (c) 2011 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/base/single_request_host_resolver.h"
 
 #include "base/compiler_specific.h"
 #include "base/logging.h"
 #include "net/base/net_errors.h"
 
 namespace net {
 
 SingleRequestHostResolver::SingleRequestHostResolver(HostResolver* resolver)
     : resolver_(resolver),
       cur_request_(NULL),
       cur_request_callback_(NULL),
       ALLOW_THIS_IN_INITIALIZER_LIST(
           callback_(this, &SingleRequestHostResolver::OnResolveCompletion)) {
   DCHECK(resolver_ != NULL);
 }
 
 SingleRequestHostResolver::~SingleRequestHostResolver() {
   Cancel();
 }
 
 int SingleRequestHostResolver::Resolve(const HostResolver::RequestInfo& info,
                                        AddressList* addresses,
                                        CompletionCallback* callback,
                                        const BoundNetLog& net_log) {
+  DCHECK(addresses);
+  DCHECK(callback);
   DCHECK(!cur_request_callback_) << "resolver already in use";
 
   HostResolver::RequestHandle request = NULL;
 
   // We need to be notified of completion before |callback| is called, so that
   // we can clear out |cur_request_*|.
   CompletionCallback* transient_callback = callback ? &callback_ : NULL;
 
   int rv = resolver_->Resolve(
       info, addresses, transient_callback, &request, net_log);
@@ skipping 23 matching lines @@
 
   // Clear the outstanding request information.
   cur_request_ = NULL;
   cur_request_callback_ = NULL;
 
   // Call the user's original callback.
   callback->Run(result);
 }
 
 }  // namespace net