MDEV-9535 Trigger doing "SET NEW.auctionStart = NOW();" on a timestamp kills MariaDB server

when doing set_field_to_new_field (from switch_to_nullable_trigger_fields())
make sure that the field we're about to change actually belongs
to the right table (otherwise we cannot dereference new_field[]
array as the wrong table might have more fields than
new_field[] has elements)

mysql-test/r/trigger_null-8605.result

@@ -334,3 +334,10 @@ a	b
 1	3
 2	4
 drop table t1, t2;
+create table t1 (a int not null);
+create table t2 (f1 int unsigned not null, f2 int);
+insert into t2 values (1, null);
+create trigger tr1 before update on t1 for each row do 1;
+create trigger tr2 after update on t2 for each row update t1 set a=new.f2;
+update t2 set f2=1 where f1=1;
+drop table t1, t2;

mysql-test/t/trigger_null-8605.test

@@ -349,3 +349,14 @@ create table t2 select a as c, b as d from t1;
 update t1 set a=(select count(c) from t2 where c+1=a+1 group by a);
 select * from t1;
 drop table t1, t2;
+
+#
+# MDEV-9535 Trigger doing "SET NEW.auctionStart = NOW();" on a timestamp kills MariaDB server.
+#
+create table t1 (a int not null);
+create table t2 (f1 int unsigned not null, f2 int);
+insert into t2 values (1, null);
+create trigger tr1 before update on t1 for each row do 1;
+create trigger tr2 after update on t2 for each row update t1 set a=new.f2;
+update t2 set f2=1 where f1=1;
+drop table t1, t2;

sql/item.cc

@@ -2381,7 +2381,7 @@ bool Item_field::update_table_bitmaps_processor(uchar *arg)
 
 static inline void set_field_to_new_field(Field **field, Field **new_field)
 {
-  if (*field)
+  if (*field && (*field)->table == new_field[0]->table)
   {
     Field *newf= new_field[(*field)->field_index];
     if ((*field)->ptr == newf->ptr)