chan_dahdi/sig_pri: Fix crash on ISDN call hangup collision.

If an ISDN call is hungup by both sides at the same time a crash could
happen.

* Added missing NULL checks for the owner channel after calling
pri_queue_pvt_cause_data() in two places.  Code after those calls need to
check the owner channel pointer for NULL before use because
pri_queue_pvt_cause_data() needs to do deadlock avoidance to lock the
owner and the owner may get hung up.

ASTERISK-21893 #close
Reported by:  Alexandr Gordeev

Change-Id: Ica3e266ebc7a894b41d762326f08653e1904bb9a