Fix access to just-closed relcache entry.

It might be impossible for this to cause a problem in non-debug builds,
since there'd be no opportunity for the relcache entry to get recycled
before the fetch.  It blows up nicely with -DRELCACHE_FORCE_RELEASE plus
valgrind, though.

Evidently introduced by careless refactoring in commit f0e44751d.
Back-patch accordingly.

Discussion: https://postgr.es/m/27543.1528758304@sss.pgh.pa.us

src/backend/commands/tablecmds.c

@@ -11922,12 +11922,12 @@ ATExecDropInherit(Relation rel, RangeVar *parent, LOCKMODE lockmode)
 	/* Off to RemoveInheritance() where most of the work happens */
 	RemoveInheritance(rel, parent_rel);
 
-	/* keep our lock on the parent relation until commit */
-	heap_close(parent_rel, NoLock);
-
 	ObjectAddressSet(address, RelationRelationId,
 					 RelationGetRelid(parent_rel));
 
+	/* keep our lock on the parent relation until commit */
+	heap_close(parent_rel, NoLock);
+
 	return address;
 }