Update libraries/joomla/cache/cache.php

Fix to "$registeredurlparams not found".

This is the same solution as in the similar bug fix https://github.com/joomla/joomla-cms/commit/0cb249a858b8da5eb5bc16fef92dd9d8bf0ad5c5

There appear to be no further instances of this problem in the libraries subdirectory.