mysql-test/r/row.result:
  Correct NULL handling in row comporison
mysql-test/t/row.test:
  Correct NULL handling in row comporison (BUG#12509)
sql/item_cmpfunc.cc:
  stop on NULL comparison only if it is allowed
sql/item_cmpfunc.h:
  support optimisation in case of processing WHERE/HAVING, where NULL and FALSE are equal