[BUGFIX] Prevent null pointer exception in BackendController

On processing a redirect parameter for fetching
the backend module it is now properly checked
whether the resolved route is a module route.

Resolves: #105228
Releases: main, 12.4
Change-Id: I2a09a9a375187c17632f5a7e3a2e3e45a3e39d77
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/86469


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Thomas Hohn <tho@gyldendal.dk>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>