qom: Crash more nicely on object_property_get_link() failure

Pass &error_abort instead of NULL where the returned value is
dereferenced or asserted to be non-null.  Drop a now redundant
assertion.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200707160613.848843-24-armbru@redhat.com>

hw/ppc/spapr_hcall.c

@@ -1655,7 +1655,8 @@ static void spapr_handle_transient_dev_before_cas(SpaprMachineState *spapr)
             continue;
         }
         drc = SPAPR_DR_CONNECTOR(object_property_get_link(drc_container,
-                                                          prop->name, NULL));
+                                                          prop->name,
+                                                          &error_abort));
 
         if (spapr_drc_transient(drc)) {
             spapr_drc_reset(drc);