HPCC-11690 Fix DEDUP,HASH crash if OOM freed empty bucket

If a roxiemem OOM event, caused an empty bucket to be cleared
(to free it's ptrtable table), then a subsequent row add to that
bucket caused a crash, referencing the rowwriter which had not
been setup.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>