When a too long field is used for a key, only a prefix part of the field is 
used. Length is reduced to the max key length allowed for storage. But if the
field have a multibyte charset it is possible to break multibyte char
sequence. This leads to the failed assertion in the innodb code and 
server crash when a record is inserted.

The make_prepare_table() now aligns truncated key length to the boundary of
multibyte char.