Cache obsolete custom field names

If a custom field name changed, custom_field_get_id_from_name() keeps
asking the database for an id over and over again (e.g. for every
change of the field in the bug history) to decide if it should be
displayed.

Thanks to user nimmich for noticing the issue and providing a patch.

Fixes #15265

core/custom_field_api.php

@@ -651,6 +651,7 @@ function custom_field_get_id_from_name( $p_field_name, $p_truncated_length = nul
 	$t_result = db_query_bound( $query, array( $c_field_name ) );
 
 	if( db_num_rows( $t_result ) == 0 ) {
+		$g_cache_name_to_id_map[$p_field_name] = false;
 		return false;
 	}