Bug 5956 - Error Code: 138 IDB-2015: Sorting length exceeded when using a view

Added checking in make_string_field function of item.cc. If thd is in VTABLE state
(infinidb query), or the table list contains InfiniDB tables, a varchar field is always created
instead of a blob field, even if the field length > CONVERT_IF_BIGGER_TO_BLOB.