Fix for bug #5888 "Triggers with nonexistent columns cause packets

out of order". (final version)

Now instead of binding Item_trigger_field to TABLE objects during
trigger definition parsing at table open, we perform pass through
special list of all such objects in trigger. This allows easily check
all references to fields in old/new version of row in trigger during
execution of CREATE TRIGGER statement (this is more courtesy for users
since we can't check everything anyway).
We also report that such reference is bad by returning error from
Item_trigger_field::fix_fields() method (instead of setup_field())
This means that if trigger is broken we will bark during trigger
execution instead of trigger definition parsing at table open.
(i.e. now we allow to open tables with broken triggers).

mysql-test/r/trigger.result

@@ -150,6 +150,10 @@ create trigger trg before delete on t1 for each row set new.i:=1;
 ERROR HY000: There is no NEW row in on DELETE trigger
 create trigger trg after update on t1 for each row set new.i:=1;
 ERROR HY000: Updating of NEW row is not allowed in after trigger
+create trigger trg before update on t1 for each row set new.j:=1;
+ERROR 42S22: Unknown column 'j' in 'NEW'
+create trigger trg before update on t1 for each row set @a:=old.j;
+ERROR 42S22: Unknown column 'j' in 'OLD'
 create trigger trg before insert on t2 for each row set @a:=1;
 ERROR 42S02: Table 'test.t2' doesn't exist
 create trigger trg before insert on t1 for each row set @a:=1;