Only call get_last_child() if the node has children

Explain that the behaviour is otherwise undefined, in the comment.

src/xmpp/xmpp_component.cpp

@@ -380,7 +380,7 @@ void XmppComponent::handle_message(const Stanza& stanza)
       // We consider this to be true, unless the error condition is
       // specified and is not in the kickable_errors set
       bool kickable_error = true;
-      if (error)
+      if (error && error->has_children())
         {
           const XmlNode* condition = error->get_last_child();
           if (kickable_errors.find(condition->get_name()) == kickable_errors.end())

src/xmpp/xmpp_stanza.hpp

@@ -77,7 +77,9 @@ public:
   XmlNode* add_child(XmlNode* child);
   XmlNode* add_child(XmlNode&& child);
   /**
-   * Returns the last of the children
+   * Returns the last of the children. If the node doesn't have any child,
+   * the behaviour is undefined. The user should make sure this is the case
+   * by calling has_children() for example.
    */
   XmlNode* get_last_child() const;
   /**