AST-2014-002: chan_sip: Exit early on bad session timers request

AST-2014-002: chan_sip: Exit early on bad session timers request

This change allows chan_sip to avoid creation of the channel and
consumption of associated file descriptors altogether if the inbound
request is going to be rejected anyway.

(closes issue ASTERISK-23373)
Reported by: Corey Farrell
Patches:
     chan_sip-earlier-st-1.8.patch uploaded by Corey Farrell (license 5909)
     chan_sip-earlier-st-11.patch uploaded by Corey Farrell (license 5909)
........

Merged revisions 410308 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 410311 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/certified/branches/11.6@410359 65c4cc65-6c06-0410-ace0-fbb531ad65f3