PJSIP: Fix assert on initial mass qualify

This fixes the MWI test regressions caused by r429127 and ensures that
contacts have non-zero qualify_frequency before attempting scheduling.
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Merged revisions 429245 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 429246 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@429247 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_pjsip/pjsip_options.c

@@ -960,7 +960,9 @@ static int qualify_and_schedule_cb(void *obj, void *arg, int flags)
 	initial_interval = contact->qualify_frequency * 1000;
 	initial_interval = (int)(initial_interval * ast_random_double());
 
+	if (contact->qualify_frequency) {
 		schedule_qualify(contact, initial_interval);
+	}
 
 	return 0;
 }