FROM_DAYS(<timestamp column>) returns different result in MariaDB comparing to MySQL: NULL vs 0000-00-00

It's rather a cosmetic difference, but I want to fix our failures on MySQL engines suite (again), and this is one of them. So, if it's intended behavior, please reassign the bug back to me so I update the result file for engines/funcs/r/date_function.

MariaDB 5.3, 5.5, 10.0:

CREATE TABLE t1(c1 TIMESTAMP NOT NULL) ENGINE=MyISAM;

INSERT INTO t1 VALUES('2008-02-29');

SELECT FROM_DAYS(c1) FROM t1;

# FROM_DAYS(c1)

# NULL

MySQL 5.1, 5.5, 5.6, MariaDB 5.2:

CREATE TABLE t1(c1 TIMESTAMP NOT NULL) ENGINE=MyISAM;

INSERT INTO t1 VALUES('2008-02-29');

SELECT FROM_DAYS(c1) FROM t1;

# FROM_DAYS(c1)

# 0000-00-00

No warnings on either version.