- Use %lx instead of %p as %p is not portable
- Don't replace ROW item with Item_null


myisam/mi_rkey.c:
  Use %lx instead of %p as %p is not portable
myisam/mi_search.c:
  Use %lx instead of %p as %p is not portable
mysql-test/r/select.result:
  More test for ROW comparison
mysql-test/t/select.test:
  More test for ROW comparison
sql/item.cc:
  We can't replace a ROW item with Item_null because:
  - Arg_comparator is still using the orignal row items and it's not good to have args[0] different from what is actually compared
  - If we are using the <=> comparator, NULL can still be compared as true
  - We would break things if we would ever use resolve_const_item() for operators like >, < etc.