parser xml BUGFIX extensions do not have to parse any data

Fixes #1816
diff --git a/src/parser_xml.c b/src/parser_xml.c
index cb237f5..74bdf6e 100644
--- a/src/parser_xml.c
+++ b/src/parser_xml.c
@@ -446,6 +446,11 @@
         nested_exts = parent->schema->exts;
     }
     LY_ARRAY_FOR(nested_exts, u) {
+        if (!nested_exts[u].def->plugin->parse) {
+            /* not an extension with parsed data */
+            continue;
+        }
+
         /* prepare the input and try to parse this extension data */
         in_ext = in_start;
         ext_parse_cb = nested_exts[u].def->plugin->parse;