gnattest: unsave list access in Test.Skeleton.Add_DT

[kilian-gebhardt]

I experienced a crash in gnattest's Test.Skeleton.Add_DT:

      loop
         exit when TP_Cur = TP_Mapping_List.No_Element;

         if TP_Mapping_List.Element (TP_Cur).TP_Name.all = TPtarg then
            exit;
         end if;

         TP_Mapping_List.Next (TP_Cur);
      end loop;

      TP := TP_Mapping_List.Element (TP_Cur);

If TP_Cur = TP_Mapping_List.No_Element, then TP_Mapping_List.Element (TP_Cur) raises a constraint error.