HPCC-11523 Fix roxie core on unloadable workunits

Queries that are suspended because dll cannot be loaded may core if called

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5944 from richardkchapman/roxie-core-missing-dll

HPCC-11523 Fix roxie core on unloadable workunits

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>