chan_iax2.c: Fix ref leak in iax2_request().

* Increased warning message format capability string buffer size in
iax2_request().

Review: https://reviewboard.asterisk.org/r/4601/
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Merged revisions 434510 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@434511 65c4cc65-6c06-0410-ace0-fbb531ad65f3