Variable is undefined please use a proper ide in the future whoever did that

https://github.com/owncloud/core/commit/76b705602629e8ef6ff19176bcd81d92c9cdcad6 fixes the error in #12681

@schiesbn @derkostka Please test and review.
No idea what L63 is assumed to do but `$name` contains the username as can be seen in L59. (`OC_User::userExists($name)`)