chan_sip: Fix incorrect use of timers

chan_sip: Fix incorrect use of timers

If update_provisional_keepalive() is called while
send_provisional_keepalive_full() is waiting on the PVT lock, then
pvt->provisional_keepalive_sched_id will be changed to a new sched_id
value by update_provisional_keepalive(), but that new sched_id then may
be overwritten with -1 by send_provisional_keepalive_full(), killing
the pvt's reference to a schedule and "leaking" the reference.

(closes issue ASTERISK-22079)
Review: https://reviewboard.asterisk.org/r/3368/
Reported by: Jamuel Starkey, Matteo, Leif Madsen, Steve Davies
Patches:
    provisional_keepalive_fix.diff uploaded by Steve Davies (license 5012)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@411088 65c4cc65-6c06-0410-ace0-fbb531ad65f3