server: silence build warning

The commit fixes this build warning:

====
src/server.c:223:8: warning: ‘privkey’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     rc = ssh_pki_export_privkey_to_pubkey(*privkey, &pubkey);
        ^
src/server.c:243:11: note: ‘privkey’ was declared here
   ssh_key privkey;
====

Signed-off-by: Luka Perkov <luka.perkov@sartura.hr>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/server.c

@@ -217,6 +217,7 @@ int ssh_get_key_params(ssh_session session, ssh_key *privkey){
         *privkey = session->srv.ecdsa_key;
         break;
       case SSH_KEYTYPE_UNKNOWN:
+      default:
         *privkey = NULL;
     }