HPCC-11140 Memory corruption if keyindex returns zero

Should not be happening, but some invalid keys are causing Roxie to restart
after heap corruptions.

The same invalid keys may also report key size mismatch on key [merger]

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5639 from richardkchapman/roxie-keysize-zero

HPCC-11140 Memory corruption if keyindex returns zero

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>