Backport from 5.6: Bug#14769820 ASSERT FLEN == LEN IN ALTER TABLE ... ADD UNIQUE KEY
Backport from 5.6: Bug#14769820 ASSERT FLEN == LEN
IN ALTER TABLE ... ADD UNIQUE KEY

A bogus debug assertion failure occurred when reporting a duplicate
key on a column prefix of a CHAR column.

This is a regression from Bug#14729221 IN-PLACE ALTER TABLE REPORTS ''
INSTEAD OF REAL DUPLICATE VALUE FOR PREFIX KEYS. The assertion is only
present when UNIV_DEBUG is defined (which it is in debug builds
starting from MySQL 5.5). It is a case of overasserting.

Fix approved by Inaam Rana on IM.

davidl-zend/zenddbi,flynn1973/mariadb-aix,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,davidl-zend/zenddbi,flynn1973/mariadb-aix,davidl-zend/zenddbi,natsys/mariadb_10.2,natsys/mariadb_10.2,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,davidl-zend/zenddbi,natsys/mariadb_10.2,flynn1973/mariadb-aix,natsys/mariadb_10.2,slanterns/server,ollie314/server,flynn1973/mariadb-aix,ollie314/server,davidl-zend/zenddbi,davidl-zend/zenddbi,flynn1973/mariadb-aix,davidl-zend/zenddbi,natsys/mariadb_10.2,flynn1973/mariadb-aix,davidl-zend/zenddbi,ollie314/server,ollie314/server,flynn1973/mariadb-aix,natsys/mariadb_10.2,flynn1973/mariadb-aix,flynn1973/mariadb-aix,ollie314/server,davidl-zend/zenddbi,davidl-zend/zenddbi,ollie314/server,ollie314/server,ollie314/server,natsys/mariadb_10.2,natsys/mariadb_10.2,ollie314/server