nvme-pci: check segement valid for SGL use

The driver needs to verify there is a payload with a command before
seeing if it should use SGLs to map it.

Fixes: 955b1b5a ("nvme-pci: move use_sgl initialization to nvme_init_iod()")
Reported-by: Paul Menzel <pmenzel+linux-nvme@molgen.mpg.de>
Reviewed-by: Paul Menzel <pmenzel+linux-nvme@molgen.mpg.de>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>