indxf: allow empty num_classes for <=r2000

e.g. r14 dxf's

Fixes GH #1068

src/in_dxf.c

@@ -1609,7 +1609,7 @@ dxf_classes_read (Bit_Chain *restrict dat, Dwg_Data *restrict dwg)
           if (!pair)
             return 1;
         }
-      else
+      else if (dwg->version >= R_2000)
         {
           LOG_ERROR ("2 CLASSES must be followed by 0 CLASS")
           DXF_RETURN_EOF (DWG_ERR_INVALIDDWG);