fix corner case in reuse of mangle options (#3062)

lib/scope.js

@@ -344,7 +344,7 @@ function next_mangled_name(scope, options, def) {
     }
     while (true) {
         name = base54(++scope.cname);
-        if (in_use[name] || !is_identifier(name) || options._reserved[name]) continue;
+        if (in_use[name] || !is_identifier(name) || options.reserved.has[name]) continue;
         if (!names[name]) break;
         holes.push(scope.cname);
     }
@@ -387,7 +387,7 @@ function _default_mangler_options(options) {
     if (!Array.isArray(options.reserved)) options.reserved = [];
     // Never mangle arguments
     push_uniq(options.reserved, "arguments");
-    options._reserved = makePredicate(options.reserved);
+    options.reserved.has = makePredicate(options.reserved);
     return options;
 }
 
@@ -453,7 +453,7 @@ AST_Toplevel.DEFMETHOD("mangle_names", function(options){
     redefined.forEach(mangle);
 
     function mangle(def) {
-        if (options._reserved[def.name]) return;
+        if (options.reserved.has[def.name]) return;
         def.mangle(options);
     }
 });
@@ -504,7 +504,7 @@ AST_Toplevel.DEFMETHOD("expand_names", function(options) {
     function rename(def) {
         if (def.global && options.cache) return;
         if (def.unmangleable(options)) return;
-        if (options._reserved[def.name]) return;
+        if (options.reserved.has[def.name]) return;
         var d = def.redefined();
         def.name = d ? d.name : next_name();
         def.orig.forEach(function(sym) {