backlight: Do not hang forever if helper pid is -1

Backlight helper PID is set to -1 by default, if for some reason it's
not set, we may end up with waitpid(-1, ...) which will hang forever.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90230
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>

src/backlight.c

@@ -523,7 +523,7 @@ void backlight_disable(struct backlight *b)
 void backlight_close(struct backlight *b)
 {
 	backlight_disable(b);
-	if (b->pid)
+	if (b->pid > 0)
 		waitpid(b->pid, NULL, 0);
 }