decode: improve Invalid preview_size error

with to/r13/from_autocad_r13/ROBOT.DWG. check against the obj->size
not some hardcoded too low limit.

But now this file fail with out of memory on outjson

src/common_entity_data.spec

@@ -321,7 +321,7 @@
             FIELD_BLL (preview_size, 160);
           }
 #endif
-        if ((long)_ent->preview_size >= 0 && _ent->preview_size < 210210)
+        if ((long)_ent->preview_size >= 0 && _ent->preview_size < obj->size)
           {
             FIELD_BINARY (preview, _ent->preview_size, 310);
           }