cosmetics.
fixed a subtle crash on Windows.

appleseed.studio would crash when deallocating a std::string allocated by appleseed.dll.  The std::string should really have been allocated by appleseed.studio, however it turns out that inline methods in DLL-exported classes are not really inline, in our case meaning the std::string was really allocated inside appleseed.dll.
Merge branch 'master' of github.com:appleseedhq/appleseed