If an object marked OF_IMSCAND is used as the template during backend
IMS revalidation, and the new object also is marked OC_F_PASS
(hit-for-pass) in vcl_backend_response, the resulting object would
(though ObjCopyAttr) have both OF_IMSCAND and OC_F_PASS. The object
could then be used again for backend IMS revalidation (since it has
the OF_IMSCAND flag), which would cause troubles as the body would by
this time be deleted.

Fix by not considering objects marked OC_F_PASS as a stale object
candidate during lookup.

Fixes: #1956