disable login again while connecting

[anzyelay]

Fixes #551

at first i input the enter key to active screen then it cant has focus on the input lineeditor. i found it occurs when you repeat to input the enter key quickly with wrong password(or empty) ,the input editor will be insensitive and cannot enter anything.

[jeremypw]

I have linked this PR to a reported issue that this is intended to fix. If this is done, then that issue will be automatically closed once the PR is merged.

[jeremypw]

@anzyelay For future reference, it is recommended to make a separate branch on your forked master for each PR you make. Please do not submit PRs consisting of a patched master branch.

[jeremypw]

You can check for some codestyle (lint) issues by installing vala-lint from https://github.com/vala-lang/vala-lint and using it to check your branch locally before pushing.