UNION doesn't return DISTINCT result for multi-byte characters
hp_hash.c:
  This piece of code was pretty wrong, looks
  like no necessary changes were made after
  cut-and-paste from fixed length segment processing.
  Itroduced two new variables safe_length1 and
  safe_length2 to remember the original lengths.
  Fixing my_charpos and set_if_smaller calls
  to pass correct parameters.
ctype_utf8.result, ctype_utf8.test:
  adding test case


heap/hp_hash.c:
   Bug#12891:
  UNION doesn't return DISTINCT result for multi-byte characters
  This piece of code was pretty wrong.
mysql-test/t/ctype_utf8.test:
  adding test case
mysql-test/r/ctype_utf8.result:
  adding test case