Add a map key hash specialisation for Null

xzyfer · xzyfer · commit ee9cd0b29838 · 2015-07-13T15:31:53.000+10:00
diff --git a/ast.hpp b/ast.hpp
@@ -1598,7 +1598,7 @@ namespace Sass {
 
     virtual size_t hash()
     {
-      return 0;
+      return -1;
     }
 
     ATTACH_OPERATIONS()
