Fixes for sass_interface (C Bindings)

Hi

I'm currently trying to get the latest Version of libsass to work with Perl CSS::Sass. Unfortunately I've hit a few obstacles on the way:

A) The current sass_interface does not seem to be in sync with the rest of the library
B) The sass_interface crashes if there is no image_path set. This could be solved by the perl implementation, but I guess it doesn't hurt to include this protection for all implementations. By the way, it was crashing with this message:

terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_S_construct null not valid

I hope this pull request is useful!

Best wishes
Marcel Greter