SMACK-230 Added synchronization since disconnect can be called from multiple threads.

git-svn-id: http://svn.igniterealtime.org/svn/repos/smack/branches/smack_3_2_0@12540 b35dd754-fafc-0310-a699-88a17e54d16e

source/org/jivesoftware/smack/XMPPConnection.java

@@ -419,7 +419,7 @@ public class XMPPConnection extends Connection {
         saslAuthentication.init();
     }
 
-    public void disconnect(Presence unavailablePresence) {
+    public synchronized void disconnect(Presence unavailablePresence) {
         // If not connected, ignore this request.
         if (packetReader == null || packetWriter == null) {
             return;