ordered data from archive tables

Archive was using wrong memory address to check if field
is NULL (after filesort, when reading record again).

mysql-test/r/archive.result:
  A test case for BUG#11764339.
mysql-test/t/archive.test:
  A test case for BUG#11764339.
storage/archive/ha_archive.cc:
  Null bytes are restored to "record" buffer, which may
  or may not be equal to record buffer for field. Check
  null bits in "record" buffer, instead of Field::null_ptr.