Make xhr transports resilient to onmessage errors

Fixes sockjs#563

Author: jcheng5

lib/transport/receiver/xhr.js

@@ -37,12 +37,13 @@ XhrReceiver.prototype._chunkHandler = function(status, text) {
     return;
   }
 
-  for (var idx = -1; ; this.bufferPosition += idx + 1) {
+  for (var idx = -1; ; ) {
     var buf = text.slice(this.bufferPosition);
     idx = buf.indexOf('\n');
     if (idx === -1) {
       break;
     }
+    this.bufferPosition += idx + 1;
     var msg = buf.slice(0, idx);
     if (msg) {
       debug('message', msg);