If an invalid varbindlist was returned by a backend we would call

If an invalid varbindlist was returned by a backend we would call
appl_request_upstream_resolve() twice where the first call would already
return a reply and free the upstream request, leading to a use after
free.
Make appl_request_downstream_free() call appl_request_upstream_resolve()
unconditionally and remove the call from appl_response().

OK tb@