parser xml BUGFIX avoid invalid memory access

Refs cesnet/libnetconf2#357
diff --git a/src/parser_xml.c b/src/parser_xml.c
index c9b6145..7db2b9a 100644
--- a/src/parser_xml.c
+++ b/src/parser_xml.c
@@ -1059,8 +1059,7 @@
             /* custom elements */
             r = lydxml_opaq_r(xmlctx, parent);
             LY_CHECK_GOTO(r, error);
-
-            no_dup = 0;
+            continue;
         }
 
 check_child: