Merge pull request #242 from igniterealtime/4.3_MUC-leave-without-nickname

fix: Prevent attempt to construct invalid address

smack-extensions/src/main/java/org/jivesoftware/smackx/muc/MultiUserChat.java

@@ -756,6 +756,10 @@ public class MultiUserChat {
         // throw.
         userHasLeft();
 
+        if (nickname == null) {
+            return;
+        }
+
         // We leave a room by sending a presence packet where the "to"
         // field is in the form "roomName@service/nickname"
         Presence leavePresence = new Presence(Presence.Type.unavailable);