fixes deadlock in chan_sip caused by usage of MASTER_CHANNEL dialplan function

(closes issue #16767)
Reported by: lmsteffan
Patches:
      deadlock_16767v3.diff uploaded by dvossel (license 671)

Review: https://reviewboard.asterisk.org/r/606/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@256319 65c4cc65-6c06-0410-ace0-fbb531ad65f3