pgsql: Fix crash with pg_partition_root

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2ab6d28d233af17987ea323e3235b2bda89b4f2e

Modified Files
--------------
src/backend/utils/adt/partitionfuncs.c       | 10 +++++-
src/test/regress/expected/partition_info.out | 50 ++++++++++++++++++++++++++++
src/test/regress/sql/partition_info.sql      | 12 +++++++
3 files changed, 71 insertions(+), 1 deletion(-)