Fix compiler warning

Clang warning: 
'this' pointer cannot be null in well-defined C++ code; comparison may
be assumed to always evaluate to false [-Wtautological-undefined-compare]



git-svn-id: http://sfntly.googlecode.com/svn/trunk/cpp/src@239 672e30a5-4c29-85ac-ac6d-611c735e0a51