reject malformed CLI parameters (#2239)

fixes #2237

lib/propmangle.js

@@ -76,7 +76,7 @@ function mangle_properties(ast, options) {
         only_cache: false,
         regex: null,
         reserved: null,
-    });
+    }, true);
 
     var reserved = options.reserved;
     if (!Array.isArray(reserved)) reserved = [];