netfilter: IDLETIMER: fix invalid deference of timer

"timer" was checked for null, but used later without being re-checked.

Change-Id: Ib4d08cd49860c9f157d1cac556705ba85cd44f4e
Reported-by: dan.carpenter@oracle.com
Signed-off-by: JP Abgrall <jpa@google.com>
Signed-off-by: engstk <eng.stk@sapo.pt>