MDEV-28266 Crash in Field_string::type_handler when calling procedures

on_table_fill_finished() should always be done at the end of open()
even if result is not Select_materialize but (for example)
Select_fetch_into_spvars.

sql/sql_cursor.cc

@@ -80,19 +80,7 @@ class Select_materialize: public select_unit
   Select_materialize(THD *thd_arg, select_result *result_arg):
     select_unit(thd_arg), result(result_arg), materialized_cursor(0) {}
   virtual bool send_result_set_metadata(List<Item> &list, uint flags);
-  bool send_eof()
-  {
-    if (materialized_cursor)
-      materialized_cursor->on_table_fill_finished();
-    return false;
-  }
-
-  void abort_result_set()
-  {
-    if (materialized_cursor)
-      materialized_cursor->on_table_fill_finished();
-  }
-
+  bool send_eof() { return false; }
   bool view_structure_only() const
   {
     return result->view_structure_only();
@@ -334,6 +322,8 @@ int Materialized_cursor::open(JOIN *join __attribute__((unused)))
     result->abort_result_set();
   }
 
+  on_table_fill_finished();
+
   return rc;
 }