MDEV-20515 multi-update tries to position updated table by null reference

Cause

Join tmp table inserts null row because of OUTER JOIN, that's
expected. Since `multi_update::prepare2()` converted
`Item_temptable_rowid` into `Item_field` (28dbdf3)
`multi_update::send_data()` accesses join tmp record directly and
treats it as a normal row ignoring null status of ref field. NULL ref
field is then treated as normal in `multi_update::do_updates()` which
tries to position updated table by reference 0.

Note that reference 0 may be valid reference and the first row of
table can be wrongly updated (see multi_update.test).

Fix

Do not add row into multi-update tmp table in case of null ref
field. Join tmp table does not have null_row status at this time (as
well as `STATUS_NULL_ROW`) and cannot be skipped by these properties
(see first comment in multi_update::send_data()). But it has all null
fields (including the ref field).

mysql-test/main/multi_update.result

@@ -1081,3 +1081,20 @@ b	c
 2	0
 drop view v1;
 drop table t0, t1,t2;
+#
+# MDEV-20515 multi-update tries to position updated table by null reference
+#
+create or replace table t1 (a int);
+insert into t1 values (0), (2);
+create or replace table t2 (b int);
+insert into t2 values (1), (2);
+select * from t1 left join t2 on a = b order by b;
+a	b
+0	NULL
+2	2
+update t1 left join t2 on a = b set b= 3 order by b;
+select * from t2;
+b
+1
+3
+drop tables t1, t2;