Only offer element nodes in element_first_child and element_next

Required because children contains other node types.

This matches the behaviour of the next_entity interface, and is safe
because existing callers (all in gvmd manage_sql_secinfo.c) always
check the node name.

util/xmlutils.c

@@ -2031,8 +2031,12 @@ element_attribute (element_t element, const gchar *name)
 element_t
 element_first_child (element_t element)
 {
-  if (element)
-    return element->children;
+  if (element) {
+    element = element->children;
+    while (element && (element->type != XML_ELEMENT_NODE))
+      element = element->next;
+    return element;
+  }
   return NULL;
 }
 
@@ -2046,8 +2050,12 @@ element_first_child (element_t element)
 element_t
 element_next (element_t element)
 {
-  if (element)
-    return element->next;
+  if (element) {
+    element = element->next;
+    while (element && (element->type != XML_ELEMENT_NODE))
+      element = element->next;
+    return element;
+  }
   return NULL;
 }