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: 73064ed ("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 3439ba9)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/nvme/ctrl.c

@@ -4333,7 +4333,13 @@ static uint16_t nvme_io_mgmt_send_ruh_update(NvmeCtrl *n, NvmeRequest *req)
     uint32_t npid = (cdw10 >> 1) + 1;
     unsigned int i = 0;
     g_autofree uint16_t *pids = NULL;
-    uint32_t maxnpid = n->subsys->endgrp.fdp.nrg * n->subsys->endgrp.fdp.nruh;
+    uint32_t maxnpid;
+
+    if (!ns->endgrp || !ns->endgrp->fdp.enabled) {
+        return NVME_FDP_DISABLED | NVME_DNR;
+    }
+
+    maxnpid = n->subsys->endgrp.fdp.nrg * n->subsys->endgrp.fdp.nruh;
 
     if (unlikely(npid >= MIN(NVME_FDP_MAXPIDS, maxnpid))) {
         return NVME_INVALID_FIELD | NVME_DNR;