Fix My Account page can't change the real name

Regression caused by a80f447 which introduced
strict comparison on return value of user_get_id_by_name. The function
returns false when user id is not found, but the code was still
comparing against 0.

Fixes #16519

core/user_api.php

@@ -265,7 +265,7 @@ function user_is_realname_unique( $p_username, $p_realname ) {
 		#  but allow it to match the current user
 		$t_target_user = user_get_id_by_name( $p_username );
 		$t_other_user = user_get_id_by_name( $p_realname );
-		if( ( 0 !== $t_other_user ) && ( $t_target_user !== $t_other_user ) ) {
+		if( ( false !== $t_other_user ) && ( $t_target_user !== $t_other_user ) ) {
 			return 0;
 		}