manager: Fix crash when appending a manager channel variable

manager: Fix crash when appending a manager channel variable

In r399887, a minor performance improvement was introduced by not allocating
the manager variable struct if it wasn't used. Unfortunately, when directly
accessing an ast_channel struct, manager assumed that the struct was always
allocated. Since this was no longer the case, things got a bit crashy.

This fixes that problem by simply bypassing appending variables if the manager
channel variable struct isn't there.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@400058 65c4cc65-6c06-0410-ace0-fbb531ad65f3