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: http://svn.asterisk.org/svn/asterisk/branches/11@410609 f38db490-d61c-443f-a65b-d21fe96a405b