Softmix: Fix crash when switching from softmix to another bridge technology.

The crash is caused by a race condition when switching between native RTP
and softmix bridging technologies.  In this situation, the bridging
technology is switched from native RTP to softmix, and then back to native
RTP fast enough that the softmix private data gets destroyed before the
softmix mixing thread gets started.

Thanks to Kinsey Moore for the crash analysis.

* Fix race condition when starting the softmix mixing thread and switching
to another bridge technology.

(closes issue ASTERISK-22678)
Reported by: John Bigelow
Patches:
      jira_asterisk_22678_v12.patch (license #5621) patch uploaded by rmudgett
Tested by: John Bigelow
........

Merged revisions 400849 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@400850 65c4cc65-6c06-0410-ace0-fbb531ad65f3