Update connection.js

Author: simllll

lib/protocol/connection.js

@@ -311,6 +311,7 @@ priority_loop:
     while (bucket.length > 0) {
       for (var index = 0; index < bucket.length; index++) {
         var stream = bucket[index];
+        if(!stream || !stream.upstream) continue;
         var frame = stream.upstream.read((this._window > 0) ? this._window : -1);
 
         if (!frame) {