[BUGFIX][INSTALL] Selecting existing database should check radio button

Due to a JavaScript error the corresponding radio button in Step 2 of
the 1-2-3 wizard is not automatically checked, when the dropdown menu
to select an existing database changes.

Change-Id: Idc7c65a268bd7dd67004a88a4c49457f11c022de
Resolves: #11765
Reviewed-on: http://review.typo3.org/2505
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Andreas Wolf
Tested-by: Andreas Wolf