Switched away from sprintf, which is prone to buffer overflows.

Most reasonable platforms have this function. If you're here because
this broke the build for you, consider adding an ifdef for your platform
and using sprintf there (but not on other platforms).

git-svn-id: https://svn.code.sf.net/p/jsoncpp/code/trunk/jsoncpp@269 1f120ed1-78a5-a849-adca-83f0a9e25bb6