[BUGFIX] Redirect on direct access to overlaid mountpoint

When the user directly accesses a page that is a mountpoint that
should be overlaid with the contents of the mounted page, he will
be redirected to the target page within the mounpoint context.

Releases: 6.3, 6.2
Resolves: #59382
Change-Id: Ie00c8210b39d2934b410d782799e4fc90efa8c5c
Reviewed-on: http://review.typo3.org/30603
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>