Nginx UI's json field can construct a directory traversal payload, causing arbitrary files to be written

Nginx UI is a web user interface for the Nginx web server. Nginx UI v2.0.0-beta.35 and earlier gets the value from the json field without verification, and can construct a value value in the form of `../../`. Arbitrary files can be written to the server, which may result in loss of permissions. Version 2.0.0-beta.26 fixes the issue.