Give zombies a safe channel driver to use.

Give zombies a safe channel driver to use.

Recent crashes from zombie channels suggests that they need a safe home to
goto.  When a masquerade happens, the physical part of the zombie channel
is hungup.  The hangup normally sets the channel private pointer to NULL.
If someone then blindly does a callback to the channel driver, a crash is
likely because the private pointer is NULL.

The masquerade now sets the channel technology of zombie channels to the
kill channel driver.

Related to the following issues:
(issue #19116)
(issue #19310)

Review: https://reviewboard.asterisk.org/r/1224/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@320796 65c4cc65-6c06-0410-ace0-fbb531ad65f3