Bug 25491: Fix for "Use of uninitialized value" in InstallAuth.pm

Bug 25491: Fix for "Use of uninitialized value" in InstallAuth.pm

This warning was thrown:
    Use of uninitialized value $info{"invalid_username_or_password"}
    in numeric eq (==) at /home/vagrant/kohaclone/C4/InstallAuth.pm
    line 387.

There is the case when hash key can be undefined in numeric comparison.

Fixed by adding additional precheck for
$info{"invalid_username_or_password"} being Perl's "true".

To test:
    1) Go to the first page of the web-installer where it asks to login.
    2) Observe the warning in the log file.
    3) Apply patch.
    4) Repeat step 1.
    7) Check that previous warning suppressed.

Mentored-by: Andrew Nugged <nugged@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>