dh: fix segfault in server

src/dh.c

@@ -686,11 +686,16 @@ int make_sessionid(ssh_session session) {
 
     ssh_string_free(num);
 #ifdef HAVE_ECDH
-    } else if (session->next_crypto->kex_type == SSH_KEX_ECDH_SHA2_NISTP256){
+  } else if (session->next_crypto->kex_type == SSH_KEX_ECDH_SHA2_NISTP256){
+    if(session->next_crypto->ecdh_client_pubkey == NULL ||
+            session->next_crypto->ecdh_server_pubkey == NULL){
+        ssh_log(session,SSH_LOG_WARNING,"ECDH parameted missing");
+        goto error;
+    }
     buffer_add_ssh_string(buf,session->next_crypto->ecdh_client_pubkey);
     buffer_add_ssh_string(buf,session->next_crypto->ecdh_server_pubkey);
 #endif
-    }
+  }
   num = make_bignum_string(session->next_crypto->k);
   if (num == NULL) {
     goto error;