Infinite import loop results in a stack overflow

[saper]

(from sass/node-sass#1315)
Test files:

=== start.scss ===

@import 'susy';

=== _susy.scss ===

@import 'susy';

Running sassc on start.scss results in a crash due to stack overflow.

Ruby says:

Error: An @import loop has been found: _susy.scss imports itself

Spec added sass/sass-spec#643