fix possible npe

src/main/java/eu/siacs/conversations/ui/ConversationFragment.java

@@ -1388,7 +1388,7 @@ public class ConversationFragment extends XmppFragment
     public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setHasOptionsMenu(true);
-        if (savedInstanceState == null) {
+        if (savedInstanceState == null && conversation != null) {
             conversation.jumpToLatest();
         }
     }