msm: null pointer dereferencing

Prevent unintended kernel NULL pointer dereferencing.

Orignal code:
  hlist_del_rcu(&event->hlist_entry);

Fix: Adding pointer check:
  if(!hlist_unhashed(&p_event->hlist_entry))
    hlist_del_rcu(&p_event->hlist_entry);

Bug: 25364034
Change-Id: Ieda6d8f4bb567827fa6c7709e9e729905c6c3882
Signed-off-by: Yuan Lin <yualin@google.com>