[BUGFIX] Installer fix crash with faulty DB data

If the database config, given in step 2 of install tool are faulty,
we recognice that for step 3 but try to get all database tables
in that state. Which will crash PHP since MySQLi changes.

You can reproduce this by using faulty DB data on step 2 in the
123-installer.

Resolves: #46932
Releases: 6.1
Change-Id: Ib6c756183a8ec23417115b7070d7e5ffae869a3a
Reviewed-on: https://review.typo3.org/19564
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn