was/client: postpone the Destroy() call when response is empty
was/client: postpone the Destroy() call when response is empty

Destroy() deletes the `this` pointer, and further access to
`response.headers` and others are illegal.  This fixes a potential
use-after-free bug which is currently not a problem because the
`WasClient` destructor is never called and we keep a pool reference.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy