io_uring/rsrc: don't put/free empty buffers

If cloning of buffers fail and we have to put the ones already grabbed,
check for NULL buffers and skip those. They used to be dummy ubufs, but
now they are just NULL and that should be checked before reaping them.

Reported-by: chase xd <sl1589472800@gmail.com>
Link: https://lore.kernel.org/io-uring/CADZouDQ7TcKn8gz8_efnyAEp1JvU1ktRk8PWz-tO0FXUoh8VGQ@mail.gmail.com/


Fixes: d50f94d7 ("io_uring/rsrc: get rid of the empty node and dummy_ubuf")
Signed-off-by: Jens Axboe <axboe@kernel.dk>