nss: tor_tls_release_socket() won't catch invalid socket value

From commit ae569299:

+  tor_socket_t sock =
+    tor_open_socket_nonblocking(AF_INET, SOCK_STREAM, IPPROTO_TCP);
+  if (!sock) {
+    log_warn(LD_NET, "Out of sockets when trying to shut down an NSS "
+             "connection");
+    return;
+  }

A socket value of 0 is a valid socket but not only that, if socket() did ran out of fd, -1 will be returned (or other errors would send back TOR_INVALID_SOCKET = -1).

So we probably want if (sock < 0) {}