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/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@434510 65c4cc65-6c06-0410-ace0-fbb531ad65f3