test/t_http_client: fix bogus assertion failure in destructor
test/t_http_client: fix bogus assertion failure in destructor

Destructor should check whether the socket has been closed already;
the FilteredSocket already did it properly, but
~HttpClientConnection() needs to do it before calling waitpid().

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