spapr_events: use QTAILQ_FOREACH_SAFE() in spapr_clear_pending_events()

QTAILQ_FOREACH_SAFE() must be used when removing the current element
inside the loop block.

This fixes a user-after-free error introduced by commit 56258174238eb
and reported by Coverity (CID 1381017).

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
