Passthru sample adapter calls vbStopMaking improperly

[Mark]

I don't believe this problem exists in other adapters, but modifying adapter should be checked (it also calls vbStopMaking).

Calling vbStopMaking from noteVbContentDone is incorrect, as the adapted_body_content method may still be called by the host. The call should probably be moved to abStopMaking.

Not doing so triggers an exception in Squid, though it is currently silently ignored, because the virgin body has been discarded before its attempted retrieval (https://github.com/squid-cache/squid/blob/1c8fc2a269c1b2eaed1dc6a43dde1e2e66cca182/src/adaptation/ecap/XactionRep.cc#L553).