Fix a bug where we tried to send events out when no sessions container was present.

This commit stops a warning message (user_data is NULL) from getting output when
manager events get sent before manager is initialized. This happens because manager
is initialized *after* modules are loaded and the act of loading modules triggers
manager events.

(issue #14974)
Reported by: pj


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@190586 65c4cc65-6c06-0410-ace0-fbb531ad65f3