Fix mangle with option keep_fnames=true for Safari.

Fixes: #1202

lib/scope.js

@@ -314,9 +314,13 @@ AST_Function.DEFMETHOD("next_mangled", function(options, def){
     // a function expression's argument cannot shadow the function expression's name
 
     var tricky_def = def.orig[0] instanceof AST_SymbolFunarg && this.name && this.name.definition();
+
+    // the function's mangled_name is null when keep_fnames is true
+    var tricky_name = tricky_def ? tricky_def.mangled_name || tricky_def.name : null;
+
     while (true) {
         var name = AST_Lambda.prototype.next_mangled.call(this, options, def);
-        if (!(tricky_def && tricky_def.mangled_name == name))
+        if (!tricky_name || tricky_name != name)
             return name;
     }
 });