Clear the login password field as soon as we've stashed the value.

While we're at it, clear the ShareRooms password field similarly -
unnecessary, since we don't return to the form, but more consistent, in
case we ever do return there.
But don't clear the login password if it wasn't really tested.

Ie, don't clear on temporary server failure or authentication
interrupted.

The process of fetching a ShareRoom doesn't have such discrimination, so
we clear whenever the fetch of a password-protected password fails.
Merge pull request #518 from SpiderOak/issue-513-clear-failed-password

Clear the login password field as soon as we've stashed the value.