fix destructuring bugs in mangle and compress (#1992)

- destructuring mangle
- destructuring array default values

fixes #1335

lib/scope.js

@@ -569,7 +569,6 @@ AST_Toplevel.DEFMETHOD("mangle_names", function(options){
     });
     this.walk(tw);
     to_mangle.forEach(function(def){
-        if (def.destructuring && !def.destructuring.is_array) return;
         def.mangle(options);
     });