nss: tor_tls_release_socket() won't catch invalid socket value
From commit ae5692994fc31cc5fa25fb5681e59e326e6c5dbe:
```
+ 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) {}`
issue