curl: use CURL_SOCKET_BAD to ensure cross-platform

Solves this error in Windows build:
        ../ext/curl/gstcurlbasesink.c:1154:14: error: comparison of unsigned
        expression in '< 0' is always false [-Werror=type-limits]
        1154 |   if (curlfd < 0) {

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8615>

subprojects/gst-plugins-bad/ext/curl/gstcurlbasesink.c

@@ -1151,7 +1151,7 @@ gst_curl_base_sink_transfer_socket_cb (void *clientp, curl_socket_t curlfd,
 
   g_assert (sink);
 
-  if (curlfd < 0) {
+  if (curlfd == CURL_SOCKET_BAD) {
     /* signal an unrecoverable error to the library which will close the socket
        and return CURLE_COULDNT_CONNECT
      */