parser: fix memory leak in error path for depth more than 1

If parser has an unfinished stanza with depth > 1, it can happen that
parser->stanza points to a child. Releasing the child doesn't free the
parent stanza.

Instead of releasing parser->stanza in parser_reset() and parser_free(),
find the top most parent and release the whole tree. Also add a test
case for this memory leak.

tests/test_stanza.c

@@ -110,6 +110,10 @@ static void test_stanza_from_string(xmpp_ctx_t *ctx)
     assert(strcmp(buf, str) == 0);
     xmpp_free(ctx, buf);
     xmpp_stanza_release(stanza);
+
+    /* Error path. */
+    stanza = xmpp_stanza_new_from_string(ctx, "<uu><uu>tt");
+    assert(stanza == NULL);
 }
 
 int main()