dup may return < 0

diff --git a/src/main/command.c b/src/main/command.c
index 252691a..ad76d79 100644 (file)
--- a/src/main/command.c
+++ b/src/main/command.c
@@ -719,7 +719,8 @@ static int command_show_xml(rad_listen_t *listener, UNUSED int argc, UNUSED char
        FILE *fp;
 
        fd = dup(listener->fd);
-       if (!fd) return 0;
+       if (fd < 0) return 0;
+
        fp = fdopen(fd, "a");
        if (!fp) {
                cprintf(listener, "ERROR: Can't dup %s\n", strerror(errno));