hw/nvme: fix null pointer access in ruh update

hw/nvme: fix null pointer access in ruh update

The Reclaim Unit Update operation in I/O Management Receive does not
verify the presence of a configured endurance group prior to accessing
it.

Fix this.

Cc: qemu-stable@nongnu.org
Fixes: 73064edfb864 ("hw/nvme: flexible data placement emulation")
Reviewed-by: Jesper Wendel Devantier <j.devantier@samsung.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
(cherry picked from commit 3439ba9c5da943d96f7a3c86e0a7eb2ff48de41c)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>