HPCC-10671 Ensure activities hash table is cleared when query load fails

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5291 from ghalliday/issue10671

HPCC-10671 Ensure activities hash table is cleared when query load fails

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>