[BUGFIX] Reports module tries to load not-installed extension

When you select a report from an extension, the reports module
will remember this when you re-open it. But if you have
uninstalled the extension in the meantime, the reports module
will throw an exception. This is fixed by checking the
$reportClass. If this is NULL then redirect to indexAction.

Change-Id: Ifad4e6ee8dc651a3d6b1360486fd5a7cb2abf111
Resolves: #48523
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/22726
Reviewed-by: Stefan Neufeind
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring