Pass correct TupDesc to ri_NullCheck() in Assert

Previous coding was passing the wrong table's tuple descriptor, which
accidentally fails to fail because no existing test case exercises a
foreign key in which the referenced attributes are further to the right
of the referencing attributes.

Add a test so that further breakage is visible.

This got broken in 16828d5c.

Discussion: https://postgr.es/m/20180403204723.fqte755nukgm42uf@alvherre.pgsql

src/backend/utils/adt/ri_triggers.c

@@ -514,7 +514,7 @@ ri_Check_Pk_Match(Relation pk_rel, Relation fk_rel,
 	bool		result;
 
 	/* Only called for non-null rows */
-	Assert(ri_NullCheck(RelationGetDescr(fk_rel), old_row, riinfo, true) == RI_KEYS_NONE_NULL);
+	Assert(ri_NullCheck(RelationGetDescr(pk_rel), old_row, riinfo, true) == RI_KEYS_NONE_NULL);
 
 	if (SPI_connect() != SPI_OK_CONNECT)
 		elog(ERROR, "SPI_connect failed");

src/test/regress/expected/foreign_key.out

@@ -1381,6 +1381,19 @@ explain (costs off) delete from t1 where a = 1;
 (10 rows)
 
 delete from t1 where a = 1;
+-- Test a primary key with attributes located in later attnum positions
+-- compared to the fk attributes.
+create table pktable2 (a int, b int, c int, d int, e int, primary key (d, e));
+create table fktable2 (d int, e int, foreign key (d, e) references pktable2);
+insert into pktable2 values (1, 2, 3, 4, 5);
+insert into fktable2 values (4, 5);
+delete from pktable2;
+ERROR:  update or delete on table "pktable2" violates foreign key constraint "fktable2_d_fkey" on table "fktable2"
+DETAIL:  Key (d, e)=(4, 5) is still referenced from table "fktable2".
+update pktable2 set d = 5;
+ERROR:  update or delete on table "pktable2" violates foreign key constraint "fktable2_d_fkey" on table "fktable2"
+DETAIL:  Key (d, e)=(4, 5) is still referenced from table "fktable2".
+drop table pktable2, fktable2;
 --
 -- Test deferred FK check on a tuple deleted by a rolled-back subtransaction
 --

src/test/regress/sql/foreign_key.sql

@@ -1020,6 +1020,16 @@ create rule r1 as on delete to t1 do delete from t2 where t2.b = old.a;
 explain (costs off) delete from t1 where a = 1;
 delete from t1 where a = 1;
 
+-- Test a primary key with attributes located in later attnum positions
+-- compared to the fk attributes.
+create table pktable2 (a int, b int, c int, d int, e int, primary key (d, e));
+create table fktable2 (d int, e int, foreign key (d, e) references pktable2);
+insert into pktable2 values (1, 2, 3, 4, 5);
+insert into fktable2 values (4, 5);
+delete from pktable2;
+update pktable2 set d = 5;
+drop table pktable2, fktable2;
+
 --
 -- Test deferred FK check on a tuple deleted by a rolled-back subtransaction
 --