hw/nvme: drop AER requests without aiocb in nvme_del_sq()

nvme_del_sq() asserted r->aiocb was always set when canceling a
queue's inflight requests. A pending Async Event Request has no
aiocb (nvme_aer() parks it without issuing any block I/O), so
deleting a queue with an outstanding AER trips the assert instead of
just dropping the request.

Cc: qemu-stable@nongnu.org
Signed-off-by: Minwoo Im <minwoo.im@samsung.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>