spapr: Assert DIMM unplug state in spapr_memory_unplug()

spapr: Assert DIMM unplug state in spapr_memory_unplug()

spapr_memory_unplug() is the last step of the hot unplug sequence.
It is indirectly called by:

 spapr_lmb_release()
  hotplug_handler_unplug()

and spapr_lmb_release() already buys us that DIMM unplug state is
present : it gets restored with spapr_recover_pending_dimm_state()
if missing.

g_assert() that spapr_pending_dimm_unplugs_find() cannot return NULL
in spapr_memory_unplug() to make this clear and silence Coverity.

Fixes: Coverity CID 1450767
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <161562021166.948373.15092876234470478331.stgit@bahia.lan>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>