res_pjsip_exten_state: PJSIPShowSubscriptionsInbound causes crash

res_pjsip_exten_state: PJSIPShowSubscriptionsInbound causes crash

Currently, it is possible for some subscriptions to get into a NULL state. When
this occurs and the PJSIPShowSubscriptionsInbound ami action is issued and a
device is subscribed for extension state then the associated subscription state
object can't be located.  The code then attempts to dereference a NULL object.
Added a NULL check to avoid the problem.

Reported by: John Bigelow

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@426779 65c4cc65-6c06-0410-ace0-fbb531ad65f3