[SECURITY] Insecure Unserialize

Failing to sanitize a cookie properly,
FEtreeview is vulnerable to insecure
unserialize.

Credits go to Pascal Jungblut who
discovered and reported this issue.

http://typo3.org/news/article/insecure-unserialize-in-extension-news-tt-news/

Change-Id: Idf17babd5252a8c9f6af4b2afeee6f8ec3dc76d8
Fixes: #55981
Reviewed-on: https://review.typo3.org/27629
Reviewed-by: Christian Zenker
Tested-by: Christian Zenker
Reviewed-by: Rupert Germann
Tested-by: Rupert Germann