obfsproxy: wrong check of return value of proto_conn_create()

In network.c:open_outbound() and network.c:open_outbound_hostname():

...
  newconn = proto_conn_create(conn->cfg);
  if (!conn) {
    log_warn("%s: failed to allocate state for outbound connection",
             conn->peername);
    bufferevent_free(buf);
    return NULL;
  }
...