Fixed int->bool conversion warning on MSVC

This pull request fixes a warning on various Visual Studio versions concerning the implicit conversion from int to bool.

MSVC issues a level 3 warning C4800: "'type' : forcing value to bool 'true' or 'false' (performance warning)"
See http://msdn.microsoft.com/en-us/library/b6801kcy.aspx for detailed information.

The alternative to fix the warnings would be a compiler-specific #pragma directive.