[BUGFIX] Install Tool: Error message gives wrong info

Based on issue #45135 the change not only includes the
check for the variable length for typo_db_password, but
for typo_db, typo_db_host and typo_db_username as well.

Resolves: #45135
Releases: 4.5,4.7,6.0,6.1
Change-Id: Id32bac0597091f8bfb823dcfb700814bb138fa12
Reviewed-on: https://review.typo3.org/18324
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn