In dea05fa4, an assertion was added
that, if a vary prediction exists as marked by vary_l, that marker
matches the actual length.

This change uncovered an inconsistency where, for waitinglist returns,
the predictive vary was cleared unconditionally (req->vary_b[2] = '\0'
in VRY_Prep()), but vary_l was not.

Consequently, before dea05fa4,
VRY_Finish() would copy more of the (possibly empty) vary spec
than necessary.

This commit fixes the inconsistency and adds specific testing
for the case.

Fixes #3858