Bug#48525: trigger changes "Column 'id' cannot be null" behaviour

CHECK_FIELD_IGNORE was treated as CHECK_FIELD_ERROR_FOR_NULL;
UPDATE...SET...NULL on NOT NULL fields behaved differently after
a trigger.

Now distinguishes between IGNORE and ERROR_FOR_NULL and save/restores
check-field options.

sql/sql_class.h

@@ -995,6 +995,7 @@ public:
   bool enable_slow_log;
   bool last_insert_id_used;
   SAVEPOINT *savepoints;
+  enum enum_check_fields count_cuted_fields;
 };