Fix xmpp_stanza_release() for cloned child-stanzas.

Break the linked-list of children before releasing a child.

Before this patch it was possible, when a child is cloned and stored
for longer than the lifetime of its parent, that its `next` pointer
points to invalid memory that was already free'd when the parent stanza
was released.

This issue exists already since the initial version of
`xmpp_stanza_release()`.

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

src/stanza.c

@@ -182,6 +182,7 @@ int xmpp_stanza_release(xmpp_stanza_t *stanza)
         while (child) {
             tchild = child;
             child = child->next;
+            tchild->next = NULL;
             xmpp_stanza_release(tchild);
         }