Home link override is making incorrect usage of hook_url_outbound_alter

Something funny is going on in front_page_url_outbound_alter()

I see that you are attempting to replace any references to in a link with the module's override. However, with the code as it is, I think it's messing with too many links here. When adding in a front page override, any url() request for a NULL value will inject this front page override path in for no reason.

To reproduce:
- Add a node path to the front page override path here: admin/config/front/home-links
- Once the page reloads, the field prefix before the field that was just entered will now contain the path you just specified in the prefix string itself, as well as the textfield you just enetered

I don't completely understand the usage of the outbound alter at the moment (fairly busy, or I'd look further into it), but the attached patch fixed the issue for me.