Avoid throwing internal null-pointer exceptions in http parser.

sdk/lib/io/http_parser.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.io;
 
 // Global constants.
 class _Const {
   // Bytes for "HTTP".
   static const HTTP = const [72, 84, 84, 80];
@@ skipping 192 matching lines @@
         },
         onPause: () {
           _paused = true;
           _pauseStateChanged();
         },
         onResume: () {
           _paused = false;
           _pauseStateChanged();
         },
         onCancel: () {
-          try {
+          if (_socketSubscription != null) {
             _socketSubscription.cancel();
-          } catch (e) {
           }
         });
     _reset();
   }
 
 
   StreamSubscription<_HttpIncoming> listen(void onData(_HttpIncoming event),
                                            {Function onError,
                                             void onDone(),
                                             bool cancelOnError}) {
@@ skipping 756 matching lines @@
   _HttpHeaders _headers;
 
   // The current incoming connection.
   _HttpIncoming _incoming;
   StreamSubscription _socketSubscription;
   bool _paused = true;
   bool _bodyPaused = false;
   StreamController<_HttpIncoming> _controller;
   StreamController<List<int>> _bodyController;
 }