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: https://gitlab.com/qemu-project/qemu/-/issues/1815
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 6c8f845)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/nvme/ctrl.c

@@ -6877,7 +6877,7 @@ static uint16_t nvme_directive_receive(NvmeCtrl *n, NvmeRequest *req)
     case NVME_DIRECTIVE_IDENTIFY:
         switch (doper) {
         case NVME_DIRECTIVE_RETURN_PARAMS:
-            if (ns->endgrp->fdp.enabled) {
+            if (ns->endgrp && ns->endgrp->fdp.enabled) {
                 id.supported |= 1 << NVME_DIRECTIVE_DATA_PLACEMENT;
                 id.enabled |= 1 << NVME_DIRECTIVE_DATA_PLACEMENT;
                 id.persistent |= 1 << NVME_DIRECTIVE_DATA_PLACEMENT;