HPCC-9139 Segfault in eclagent using BLOB fields

I think this would only affect code that read indexes which were marked as
locally sorted.

Also update regression test to test blob support a little more.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4237 from richardkchapman/blob-core

HPCC-9139 Segfault in eclagent using BLOB fields

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>