Fix crash in ts_node_field_name_for_child when no children are present

Author: slackner

lib/src/node.c

@@ -571,6 +571,10 @@ TSNode ts_node_child_by_field_id(TSNode self, TSFieldId field_id) {
 
 const char *ts_node_field_name_for_child(TSNode self, uint32_t child_index) {
   const TSFieldMapEntry *field_map_start = NULL, *field_map_end = NULL;
+  if (!ts_node_child_count(self)) {
+    return NULL;
+  }
+
   ts_language_field_map(
     self.tree->language,
     ts_node__subtree(self).ptr->production_id,