Null references to request object on socket errors.

Regarding #3199 and #3179 and issues seen in production.
Hopefully this fixes them.

lib/http.js

@@ -1209,6 +1209,13 @@ ClientRequest.prototype.onSocket = function(socket) {
         // fire on the request.
         req.emit('error', createHangUpError());
       }
+
+      // Nothing more to be done with this req, since the socket
+      // is closed, and we've emitted the appropriate abort/end/close/error
+      // events.  Disavow all knowledge, and break the references to
+      // the variables trapped by closures and on the socket object.
+      req = null;
+      socket._httpMessage = null;
     }
     socket.on('close', closeListener);