HPCC-10561 Fix issues highlighted by coverity scan

Add assert to ensure that invalid parameter combinations do not
crash. I don't believe this could ever happen, but this shuts up
coverity (and is good practice).

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5372 from richardkchapman/roxie-coverity

HPCC-10561 Fix issues highlighted by coverity scan

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>