sanity check that conn exists on connection (#637)

This prevents an edge case where null is passed (and aborts the server) on a new connection if all handles are occupied.  I am able to reliably trigger this by rapidly refreshing on a page in development.

Author: jskz

lib/Server.js

@@ -363,9 +363,9 @@ Server.prototype.listen = function() {
 		}
 	});
 	sockServer.on("connection", function(conn) {
+		if(!conn) return;
 		this.sockets.push(conn);
 
-		// Remove the connection when it's closed
 		conn.on("close", function() {
 			var connIndex = this.sockets.indexOf(conn);
 			if(connIndex >= 0) {