Prevent redirections to front page path after welcome back message

I do not exactly recall in which issue was discussed, or maybe it was something I had in mind, but oh well, here's the issue to cover this f.r.

When a user visits front page and autologin is performed, the user is redirected to whatever there is the front page path, often "node", but it could be anything.

The attached patch checks if the user is on the front page and if do, redirects to the site root.