http: Do not free the wrong parser on socket close

This appears to fix #4673.  That bug is very hard to reproduce, so it's
hard to tell for certain, but this approach is more correct anyway.

Hat-tip: @dougwilson

lib/http.js

@@ -1788,7 +1788,8 @@ function connectionListener(socket) {
   function serverSocketCloseListener() {
     debug('server socket close');
     // mark this parser as reusable
-    freeParser(parser);
+    if (this.parser)
+      freeParser(this.parser);
 
     abortIncoming();
   }