Bug#12371 executing prepared statement fails (illegal mix of collations)
  After review fixes.


sql/item.cc:
  Bug#12371 executing prepared statement fails (illegal mix of collations)
  After review fixes.
mysql-test/t/ctype_utf8.test:
  Bug#12371 executing prepared statement fails (illegal mix of collations)
  After review fixes.
mysql-test/r/ctype_utf8.result:
  Bug#12371 executing prepared statement fails (illegal mix of collations)
  After review fixes.