chan_iax2: Fix a crash when using chan_iax2 jitterbuffer settings

Caused by format changes in Asterisk 13

ASTERISK-24265 #close
Reported by: Dafi Ni
Review: https://reviewboard.asterisk.org/r/3999/
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Merged revisions 423524 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@423526 65c4cc65-6c06-0410-ace0-fbb531ad65f3