#44 Fix for -Werror=format-security issue

[Tom 'spot' Callaway]

Hello,

I'm currently the maintainer for iaxclient in Fedora. Recently, Fedora
began an effort to build all of our packages with the
-Werror=format-security compiler option.

For more details, please see:
https://bugzilla.redhat.com/show_bug.cgi?id=1037126

This required one minor patch for iaxclient (based on 2.1 beta3), please apply it for future releases. I waive all copyright applicable on the change, and where that is impossible, grant a license for you to use it under the same license
terms as the rest of iaxclient.