res_pjsip_keepalive: Don't crash if PJSIP module is not loaded.

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Merged revisions 431555 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@431556 65c4cc65-6c06-0410-ace0-fbb531ad65f3