Problem:

When the VALUES() function is inappropriately used in the SET stmt the server
exits.  

set port = values(v);

This happens because the values(v) will be parsed as an Item_insert_value by
the parser.  Both Item_field and Item_insert_value return the type as
FIELD_ITEM.  But for Item_insert_value the field_name member is NULL.  In
set_var constructor, when the type of the item is FIELD_ITEM we try to access
the non-existent field_name. 

The class hierarchy is as follows:
Item -> Item_ident -> Item_field -> Item_insert_value

The Item_ident::field_name is NULL for Item_insert_value.  

Solution:

In the parsing stage, in the set_var constructor if the item type is
FIELD_ITEM and if the field_name is non-existent, then it is probably
the Item_insert_value.  So leave it as it is for later evaluation.

rb://2004 approved by Roy and Norvald.