Fix reported position when no token can be parsed

Finally fix #979

Author: mgreter

parser.cpp

@@ -143,9 +143,8 @@ namespace Sass {
       }
       else {
         lex< css_whitespace >();
-        before_token = after_token;
         if (position >= end) break;
-        error("invalid top-level expression", pstate);
+        error("invalid top-level expression", after_token);
       }
       lex< optional_spaces >();
     }