HPCC-11894 Roxie can core in superfile2 test

Obscure set of circumstances that wold actually cause the core, but basically
looking in the wrong place for exists() check.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6222 from richardkchapman/roxie-exists

HPCC-11894 Roxie can core in superfile2 test

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>