delete_returning test fails in MDEV-8646 tree.

Crashing queries:

CREATE TABLE t1 (i1 int);

INSERT INTO t1 VALUES (1),(2);

CREATE TABLE t2 (i2 int);

INSERT INTO t2 VALUES (1),(2);

DELETE FROM t1 ORDER BY i1 RETURNING ( SELECT i2 FROM t2 );

The reason the last one crashes is that the execution code first calls:

JOIN::cleanup (this=0x7fff64065c68, full=true)

and then it calls

JOIN::exec (this=0x7fff64065c68)

that is, attempts to execute a join where it already did a cleanup.