Fix crash with pg_partition_root

Trying to call the function with the top-most parent of a partition tree
was leading to a crash.  In this case the correct result is to return
the top-most parent itself.

Reported-by: Álvaro Herrera
Author: Michael Paquier
Reviewed-by: Amit Langote
Discussion: https://postgr.es/m/20190322032612.GA323@alvherre.pgsql
