hw/ppc: check if spapr_drc_index() returns NULL in spapr_nvdimm.c

hw/ppc: check if spapr_drc_index() returns NULL in spapr_nvdimm.c

spapr_nvdimm_flush_completion_cb() and flush_worker_cb() are using the
DRC object returned by spapr_drc_index() without checking it for NULL.
In this case we would be dereferencing a NULL pointer when doing
SPAPR_NVDIMM(drc->dev) and PC_DIMM(drc->dev).

This can happen if, during a scm_flush(), the DRC object is wrongly
freed/released (e.g. a bug in another part of the code).
spapr_drc_index() would then return NULL in the callbacks.

Fixes: Coverity CID 1487108, 1487178
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <20220409200856.283076-2-danielhb413@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>