The change for improving the handling of the target CQE posting
inadvertently dropped the NULL check for the submitter task on the target
ring, reinstate that.

Fixes: 0617bb50 ("io_uring/msg_ring: improve handling of target CQE posting")
Reported-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>