Fix NPE on WebSocket close, close AsyncHttpClient#1366

Motivation:

We always try to release buffered frames when WebSocket gets closed.
But the buffer might be null at this time (no frame was buffered or
they were handled once WebSocket was open).
We currently crash with NPE.

Modification:

Check for null frame buffer when trying to release.

Result:

No more NPE

Author: slandelle

client/src/main/java/org/asynchttpclient/netty/ws/NettyWebSocket.java

@@ -81,8 +81,10 @@ public void bufferFrame(WebSocketFrame frame) {
     }
 
     private void releaseBufferedFrames() {
-        for (WebSocketFrame frame : bufferedFrames) {
-            frame.release();
+        if (bufferedFrames != null) {
+            for (WebSocketFrame frame : bufferedFrames) {
+                frame.release();
+            }
         }
     }