InnoDB. This fixes also a second part of the same problem with prefix keys
on a multibyte string column for InnoDB.


innobase/rem/rem0cmp.c:
  Fixed unique prefix key or prefix key using multibyte character set bugs for
  InnoDB (BUG #4521).
  
  The unique key with a prefix of N appears index 3*N bytes of the column not N
  characters. If you had two records which have the same first N characters 
  but differ in the first 3*N bytes, then you couldn't select the records using 
  an equality test.