Otherwise, subsequent commands in the same transaction see the wrong
partition descriptor.

Amit Langote.  Reported by Tomas Vondra and David Fetter.  Reviewed
by me.

Discussion: http://postgr.es/m/22dd313b-d7fd-22b5-0787-654845c8f849%402ndquadrant.com
Discussion: http://postgr.es/m/20161215090916.GB20659%40fetter.org