HPCC-9350 - Fix DISTRIBUTE,SKEW assert

gh-4332 introduced a regression, where by DISTRIBUTE,SKEW would
assert during destruction.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Community Edition 3.10.8-9 Gold

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>