Crash fix for a case where the EndCallback is not set and used

Here is the stack trace
```
java.lang.RuntimeException: java.lang.NullPointerException
       at com.koushikdutta.async.AsyncServer.runLoop(SourceFile:789)
       at com.koushikdutta.async.AsyncServer.run(SourceFile:603)
       at com.koushikdutta.async.AsyncServer.access$700(SourceFile:37)
       at com.koushikdutta.async.AsyncServer$13.run(SourceFile:552)
Caused by: java.lang.NullPointerException
       at com.koushikdutta.async.BufferedDataEmitter.onDataAvailable(SourceFile:36)
       at com.koushikdutta.async.AsyncSSLSocketWrapper.handleHandshakeStatus(SourceFile:251)
       at com.koushikdutta.async.AsyncSSLSocketWrapper.write(SourceFile:366)
       at com.koushikdutta.async.AsyncSSLSocketWrapper.handleHandshakeStatus(SourceFile:247)
       at com.koushikdutta.async.AsyncSSLSocketWrapper.handshake(SourceFile:112)
       at com.koushikdutta.async.http.AsyncSSLSocketMiddleware.tryHandshake(SourceFile:82)
       at com.koushikdutta.async.http.AsyncSSLSocketMiddleware$2.onConnectCompleted(SourceFile:95)
       at com.koushikdutta.async.AsyncServer.runLoop(SourceFile:786)
       at com.koushikdutta.async.AsyncServer.run(SourceFile:603)
       at com.koushikdutta.async.AsyncServer.access$700(SourceFile:37)
       at com.koushikdutta.async.AsyncServer$13.run(SourceFile:552)```