tcp: Pass NULL as hostname to getaddrinfo if the string is empty

This gives you the proper v4 or v6 version of the "any address",
allowing receiving connections on any address on the machine.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/tcp.c

@@ -65,6 +65,9 @@ static int tcp_open(URLContext *h, const char *uri, int flags)
     snprintf(portstr, sizeof(portstr), "%d", port);
     if (listen_socket)
         hints.ai_flags |= AI_PASSIVE;
+    if (!hostname[0])
+        ret = getaddrinfo(NULL, portstr, &hints, &ai);
+    else
         ret = getaddrinfo(hostname, portstr, &hints, &ai);
     if (ret) {
         av_log(h, AV_LOG_ERROR,