istream_oo: move istream_deinit() call to destructor
istream_oo: move istream_deinit() call to destructor

Give it a virtual destructor so other implementations can add more
code.

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