HPCC-9364 Fix assert(mergeOrder) on stepped index read in a child query

mergeOrder incorrectly cleared per child query (but only set at activity
creation time).

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4841 from richardkchapman/mergeorderAssert

HPCC-9364 Fix assert(mergeOrder) on stepped index read in a child query

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>