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.