Support for NULL components was incomplete for row comparison,
fixed.  Added support for abort_on_null at compare_row() like
in 5.x


sql/item_cmpfunc.h:
  Bug#27704: incorrect comparison of rows with NULL components
  Added support for abort_on_null at Item_bool_func2
  like in 5.x
sql/item_cmpfunc.cc:
  Bug#27704: incorrect comparison of rows with NULL components
  Support for NULL components was incomplete for row comparison,
  fixed. Added support for abort_on_null at compare_row() like
  in 5.x
mysql-test/t/row.test:
  Test case updated for Bug#27704 (incorrect comparison 
  of rows with NULL components)
mysql-test/r/row.result:
  Test case updated for Bug#27704 (incorrect comparison 
  of rows with NULL components)
mysql-test/r/subselect.result:
  Test case updated for Bug#27704 (incorrect comparison 
  of rows with NULL components)