res_pjsip_pubsub: Set the body generation result to 0 for a valid path

The result of the "ast_sip_pubsub_generate_body_content" was not
set/initialized.  Consequently, the nominal path potentially returned
an invalid value, thus not sending mwi notifications.
........

Merged revisions 412074 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@412075 65c4cc65-6c06-0410-ace0-fbb531ad65f3