[BUGFIX] Do not throw away active session

Previously an active and valid session was dismissed, if login
credentials are sent again.

Now we do not start the user authentication if we have a valid session.

This also fixes weird side effects during backend login, when
it says that token is not valid.

Resolves: #76995
Releases: master, 7.6
Change-Id: Ia070493eb99ff395c67e0ac40e85b5e8fe7debd3
Reviewed-on: https://review.typo3.org/48865


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>