pgsql: Fix use-after-free bug when renaming constraints

Fix use-after-free bug when renaming constraints

This is an oversight from recent commit b13fd344.  While on it, tweak
the previous test with a better name for the renamed primary key.

Detected by buildfarm member prion which forces relation cache release
with -DRELCACHE_FORCE_RELEASE.  Back-patch down to 9.4 as the previous
commit.

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/91fc2a08830139d9c11b189a2407cfdf113f550f

Modified Files
--------------
src/backend/commands/tablecmds.c          | 4 ++--
src/test/regress/expected/alter_table.out | 2 +-
src/test/regress/sql/alter_table.sql      | 2 +-
3 files changed, 4 insertions(+), 4 deletions(-)