Fix a bug: returning a variable which has not been assigned in a rule causes segfault.

git-svn-id: svn://irodssvn.ucsd.edu/trunk@5372 ea137bd6-aa86-47b6-b90d-b101d9f19a90
fix a small bug in the previous checkin

git-svn-id: svn://irodssvn.ucsd.edu/trunk@5373 ea137bd6-aa86-47b6-b90d-b101d9f19a90