HPCC-9393 ATMOST stats inaccurate

Uninitialized variable means that the stats reported for keyed joins that are
reset but never started will be random.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4425 from richardkchapman/atmost-stats-uninit

HPCC-9393 ATMOST stats inaccurate

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>