This update tweaks the SELECT function, at the point commented

This update tweaks the SELECT function, at the point commented
"make sure the target relation is in the join tree".

The new logic should be equivalent to the old, except that
the old code potentially invokes undefined behavior by
reading through a null pointer.

git-svn-id: svn://svn.open-ils.org/ILS/trunk@12054 dcc99617-32d9-48b4-a31d-7c20da2025e4