innobase/row/row0sel.c:
  Fix for BUG#5401: Don't reset the entire NULL mask, only set/clear bits for particular fields.
    This is necessary because index_merge ROR-intersection uses simulatenous scans on several keys
    with the same row buffer.
mysql-test/r/index_merge_innodb.result:
  Test for BUG#5401
mysql-test/t/index_merge_innodb.test:
  Test for BUG#5401