MDEV-9428 NO_AUTO_VALUE_ON_ZERO is ignored when a trigger before insert is defined

Don't compare "field == table->next_number_field" because the field
can be special nullable field copy created by the trigger.
Compare field_index values instead.

mysql-test/r/trigger_null-8605.result

@@ -309,3 +309,16 @@ a	b	c
 1	1	1
 2	5	3
 drop table t1;
+set session sql_mode ='no_auto_value_on_zero';
+create table t1 (id int unsigned auto_increment primary key);
+insert t1 values (0);
+select * from t1;
+id
+0
+delete from t1;
+create trigger t1_bi before insert on t1 for each row begin end;
+insert t1 values (0);
+select * from t1;
+id
+0
+drop table t1;

mysql-test/t/trigger_null-8605.test

@@ -325,3 +325,17 @@ insert t1 values (9, 9, 2);
 insert t1 (a,c) values (9, 3);
 select * from t1;
 drop table t1;
+
+#
+# MDEV-9428 NO_AUTO_VALUE_ON_ZERO is ignored when a trigger before insert is defined
+#
+set session sql_mode ='no_auto_value_on_zero';
+create table t1 (id int unsigned auto_increment primary key);
+insert t1 values (0);
+select * from t1;
+delete from t1;
+create trigger t1_bi before insert on t1 for each row begin end;
+insert t1 values (0);
+select * from t1;
+drop table t1;
+

sql/sql_base.cc

@@ -8962,6 +8962,9 @@ fill_record(THD *thd, TABLE *table, Field **ptr, List<Item> &values,
   Item *value;
   Field *field;
   bool abort_on_warning_saved= thd->abort_on_warning;
+  uint autoinc_index= table->next_number_field
+                        ? table->next_number_field->field_index
+                        : ~0U;
   DBUG_ENTER("fill_record");
 
   if (!*ptr)
@@ -8987,7 +8990,7 @@ fill_record(THD *thd, TABLE *table, Field **ptr, List<Item> &values,
     DBUG_ASSERT(field->table == table);
 
     value=v++;
-    if (field == table->next_number_field)
+    if (field->field_index == autoinc_index)
       table->auto_increment_field_not_null= TRUE;
     if (field->vcol_info && 
         value->type() != Item::DEFAULT_VALUE_ITEM &&