HTTP Agent should not die on evil server

Closes GH-680.

lib/http.js

@@ -1244,9 +1244,15 @@ Agent.prototype._establishNewConnection = function() {
   parser.onIncoming = function(res, shouldKeepAlive) {
     debug('AGENT incoming response!');
 
-    var req = socket._httpMessage;
-    assert(req);
+    // If we're receiving a message but we don't have a corresponding
+    // request - then somehow the server is seriously messed up and sending
+    // multiple responses at us. In this case we'll just bail.
+    if (!socket._httpMessage) {
+      socket.destroy();
+      return;
+    }
 
+    var req = socket._httpMessage;
     req.res = res;
 
     // Responses to HEAD requests are AWFUL. Ask Ryan.