Problem: a flaw (derefencing a NULL pointer) in the LIKE optimization
code may lead to a server crash in some rare cases.

Fix: check the pointer before its dereferencing.


mysql-test/r/func_like.result:
  Fix for bug #54575: crash when joining tables with unique set column
    - test result.
mysql-test/t/func_like.test:
  Fix for bug #54575: crash when joining tables with unique set column
    - test case.
sql/item_cmpfunc.cc:
  Fix for bug #54575: crash when joining tables with unique set column
  - check res2 buffer pointer before its dereferencing 
    as it may be NULL in some cases.