Commit 4e5fe9ad introduced this
problem.  Also add a test so it doesn't get broken again.

Report by Rushabh Lathia.  Fix by Amit Langote.  Reviewed by Rushabh
Lathia and Amul Sul.  Tweaked by me.

Discussion: http://postgr.es/m/CAGPqQf0Y1iJyk4QJBdMf=pS9i6Q0JUMM_h5-qkR3OMJ-e04PyA@mail.gmail.com