msm: null pointer dereferencing

Prevent unintended kernel NULL pointer dereferencing.

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: Ib13a7400d4a36a4b08b0afc9b7d69c6027e741b6
Signed-off-by: Yuan Lin <yualin@google.com>