segfault after SPINLOCK schedule delete

segfault after SPINLOCK schedule delete

Using the SPINLOCK schedule delete macro can result in the iax_pvt lock
being given up.  This makes it possible for the iax_pvt to dissappear
when we thought we held the mutex the entire time.  To resolve this, the
iax_pvt's ref count is incremented.

(closes issue #15377)
Reported by: aragon
Patches:
      iax_spin_issue_1.4.diff uploaded by dvossel (license 671)
Tested by: aragon, dvossel

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@204067 65c4cc65-6c06-0410-ace0-fbb531ad65f3