rng: initialize file descriptor to -1

The file descriptor is never initialized to -1, which makes rng-random
close stdin if an object is created and immediately destroyed.  If we
change it to -1, we also need to protect qemu_set_fd_handler from
receiving a bogus file descriptor.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NIgor Mammedov <imammedo@redhat.com>
Tested-by: NIgor Mammedov <imammedo@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

backends/rng-random.c

@@ -123,15 +123,15 @@ static void rng_random_init(Object *obj)
                             NULL);
 
     s->filename = g_strdup("/dev/random");
+    s->fd = -1;
 }
 
 static void rng_random_finalize(Object *obj)
 {
     RndRandom *s = RNG_RANDOM(obj);
 
-    qemu_set_fd_handler(s->fd, NULL, NULL, NULL);
-
     if (s->fd != -1) {
+        qemu_set_fd_handler(s->fd, NULL, NULL, NULL);
         qemu_close(s->fd);
     }