Fix possible bug with handling empty reference combinators

Fixes #2665

Author: xzyfer

src/inspect.cpp

@@ -1042,7 +1042,7 @@ namespace Sass {
  case Complex_Selector::REFERENCE:
  append_mandatory_space();
  append_string("/");
- c->reference()->perform(this);
+ if (c->reference()) c->reference()->perform(this);
  append_string("/");
  append_mandatory_space();
  break;