Fix wrong comparison

We compare the listen() function pointer to 0 instead of the listen_fd.
This "works" because it is an extension. Using -Wall -Wextra -pedantic
reveals the error:

    main.c:456:14: warning: ordered comparison between pointer and zero ('int (*)(int, int)' and 'int') is an extension [-Wpedantic]
      456 |   if (listen >= 0) {
          |       ~~~~~~ ^  ~

Fix to compare listen_fd, and use -1 since this is is the sentinel value
meaning we failed to create the socket.

Signed-off-by: Nir Soffer <[email protected]>

Author: nirs

main.c

@@ -453,7 +453,7 @@ int main(int argc, char *argv[]) {
   if (iface != NULL) {
     stop(&state, iface);
   }
-  if (listen >= 0) {
+  if (listen_fd != -1) {
     close(listen_fd);
   }
   if (pid_fd != -1) {