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)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@410308 f38db490-d61c-443f-a65b-d21fe96a405b