Throw a NotConnectedException on disconnect()

if not connected. For consistent behavior, so that it is always
thrown. Previously it may be thrown or not.

smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java

@@ -596,7 +596,7 @@ public abstract class AbstractXMPPConnection implements XMPPConnection {
      */
     public synchronized void disconnect(Presence unavailablePresence) throws NotConnectedException {
         if (!isConnected()) {
-            return;
+            throw new NotConnectedException();
         }
 
         sendPacket(unavailablePresence);