Check for 'null' to avoid NPE in Socks5Proxy

Check if serverSocket is null before calling isClosed(), as otherwise
the resulting NPE will cause an endless loop.

Thanks to Michael Grafl for reporting.

Fixes SMACK-707.

smack-extensions/src/main/java/org/jivesoftware/smackx/bytestreams/socks5/Socks5Proxy.java

@@ -387,7 +387,7 @@ public class Socks5Proxy {
 
                 try {
 
-                    if (Socks5Proxy.this.serverSocket.isClosed()
+                    if (Socks5Proxy.this.serverSocket == null || Socks5Proxy.this.serverSocket.isClosed()
                                     || Thread.currentThread().isInterrupted()) {
                         return;
                     }