User selection of an invalid style causes a general error

User selection of a style that is not installed causes a general error of "Could not get style data." This can easily happen if a mod allows the ability for users to select a custom style for a specific area or the users are allowed to specify their own styles via the style parameter (by a slight modification of the code).

This could easily be avoided with the following change:
// User has wrong style
if (!$this->theme && $style == $this->data['user_style'])

// User has wrong style
if (!$this->theme)

I can make a PR for this, but I'm not sure how it would be wanted since session.php was split into two files for 3.1. Should it be for develop-olympus and then be handled by whoever does the merge for 3.1?