manager: fix memory leak in manager_add_filter function

(closes issue ASTERISK-23420)
Reported by: Etienne Lessard
Patches:
    manager_eventfilter_leak uploaded by Etienne Lessard (license 6394)


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@410609 65c4cc65-6c06-0410-ace0-fbb531ad65f3