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: 4aedb705

 ("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>