made BBPgetdesc() more resistant against illegal bat ids
made BBPgetdesc() more resistant against illegal bat ids

invoking [rename] multiplex with a bat(nil) could cause a SEGV

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