ESCAPE argument might be empty string. It leads
to server crash under some circumstances.
The fix:
-added check if ESCAPE argument result is not empty string

mysql-test/r/ctype_latin1.result:
  test case
mysql-test/t/ctype_latin1.test:
  test case
sql/item_cmpfunc.cc:
  -added check if ESCAPE argument result is not empty string