Preventing the server to crash because of a NPE. Issue #224

[raydelto]

NullPointerException is making the server to crash. This is also a security breach because an intruder could make your server to crash by simply connecting and start typing random characters. This commit would fix this.

[threat]

@raydelto Would it be possible to get steps to reproduce the issue that this pull fixes?

[raydelto]

Hi @threat these are are the steps for reproducing the issue:

1- Run the ChatServer Example provided within the Java-WebSoket build distribution.
2- Connect via telnet to the port where Java-WebSocket is listening (8887 by default).
3- Type random sequences of characters and line breaks (\n by hitting enter).
4- The server will stop answering requests(because it crashed due to a null pointer exception)

This is the stack trace:
java.lang.NullPointerException
at org.java_websocket.SocketChannelIOHelper.batch(SocketChannelIOHelper.
java:64)
at org.java_websocket.server.WebSocketServer.run(WebSocketServer.java:35
2)
at java.lang.Thread.run(Thread.java:745)

[threat]

Thanks @raydelto.

Just some things I've noticed:

1. Might be worth catching specific NullPointerException instead of plain Exception
2. The server no longer crashes after this fix but I'm wondering if the exception should even occur at all?

[raydelto]

@threat

1. I agree.
   2A. Someone might open a browser and open the URL http://localhost:8887 , which would make the error to occur.
   2B. Someone with bad intentions might intentionally exploit this issue in order to make the server stop answering requests, it will make the WebSocket server unavailable.

[threat]

@raydelto That makes sense. Maybe we could even introduce a new exception specifically for that case...