fixed nil-str handling in ASCIIadt_toStr.
fixed nil-str handling in ASCIIadt_toStr.

what about nil-str handling in ASCIIadt_frStr?

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb