nvme-pci: add missing condition check for existence of mapped data

nvme_map_data() is called when request has physical segments, hence
the nvme_unmap_data() should have same condition to avoid dereference.

Fixes: 4aedb70543 ("nvme-pci: split metadata handling from nvme_map_data / nvme_unmap_data")
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Nitesh Shetty <nj.shetty@samsung.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>

drivers/nvme/host/pci.c

@@ -863,7 +863,8 @@ static blk_status_t nvme_prep_rq(struct nvme_dev *dev, struct request *req)
 	nvme_start_request(req);
 	return BLK_STS_OK;
 out_unmap_data:
-	nvme_unmap_data(dev, req);
+	if (blk_rq_nr_phys_segments(req))
+		nvme_unmap_data(dev, req);
 out_free_cmd:
 	nvme_cleanup_cmd(req);
 	return ret;