static code analyzer complain:

device_tree.c:155:18: warning: Null pointer passed as an argument to a 'nonnull' parameter
    while ((de = readdir(d)) != NULL) {
                 ^~~~~~~~~~

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>