pthread_join to assure the thread is really gone

(closes issue #15465)
Reported by: fnordian
Patches:
      bridging.patch uploaded by fnordian (license 110)
Tested by: lmadsen, fnordian, peterh

Review: https://reviewboard.asterisk.org/r/679/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@266877 65c4cc65-6c06-0410-ace0-fbb531ad65f3