daemon: fix DH params generation
Issue generated from Tuleap's migration script. Originally submitted by: Guillaume Roguez (guillaume)
tls::DhParams::generate() is called by RingAccount::loadDhParams() but its result (a tls::DhParams instance)
is not checked and could be empty (default constructed instance).
In such case, the save-in-file code below call the serialize() method of DhParams that crashes when