Prevents ?do=addlink from generating a 404 if the user is not logged in
Fixes https://github.com/shaarli/Shaarli/issue/47
Merge pull request #61 from pikzen/shaarli47

Prevents ?do=addlink from generating a 404 if the user is not logged in
Fixes https://github.com/sebsauvage/Shaarli/issues/141
Fixes https://github.com/shaarli/Shaarli/issues/47