Fix #1742 - same expression on both sides of &&

Author: saper

src/ast.cpp

@@ -793,7 +793,7 @@ namespace Sass {
       if (lhs_tail->combinator() != rhs_tail->combinator()) return false;
       if (lhs_tail->head() && !rhs_tail->head()) return false;
       if (!lhs_tail->head() && rhs_tail->head()) return false;
-      if (lhs_tail->head() && lhs_tail->head()) {
+      if (lhs_tail->head() && rhs_tail->head()) {
         if (!lhs_tail->head()->is_superselector_of(rhs_tail->head())) return false;
       }
     }