hw/nvme: fix null pointer access in directive receive

nvme_directive_receive() does not check if an endurance group has been
configured (set) prior to testing if flexible data placement is enabled
or not.

Fix this.

Cc: qemu-stable@nongnu.org
Resolves: #1815
Fixes: 73064edf

 ("hw/nvme: flexible data placement emulation")
Reviewed-by: Jesper Wendel Devantier <j.devantier@samsung.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>