Clear CurrentResourceOwner earlier in CommitTransaction.

Alexander reported a crash with repeated create + drop database, after
the ResourceOwner rewrite (commit b8bff07daa). That was fixed by the
previous commit, but it nevertheless seems like a good idea clear
CurrentResourceOwner earlier, because you're not supposed to use it
for anything after we start releasing it.

Reviewed-by: Alexander Lakhin
Discussion: https://www.postgresql.org/message-id/11b70743-c5f3-3910-8e5b-dd6c115ff829%40gmail.com

src/backend/access/transam/xact.c

@@ -2309,6 +2309,7 @@ CommitTransaction(void)
 	CallXactCallbacks(is_parallel_worker ? XACT_EVENT_PARALLEL_COMMIT
 					  : XACT_EVENT_COMMIT);
 
+	CurrentResourceOwner = NULL;
 	ResourceOwnerRelease(TopTransactionResourceOwner,
 						 RESOURCE_RELEASE_BEFORE_LOCKS,
 						 true, true);
@@ -2374,7 +2375,6 @@ CommitTransaction(void)
 	AtEOXact_LogicalRepWorkers(true);
 	pgstat_report_xact_timestamp(0);
 
-	CurrentResourceOwner = NULL;
 	ResourceOwnerDelete(TopTransactionResourceOwner);
 	s->curTransactionOwner = NULL;
 	CurTransactionResourceOwner = NULL;