Fix /wp added to sites admin url when multisite is not a subdomain install

[ptrckvzn]

Proposed fix for #1

This conditional won't append /wp to admin url when not a subdomain install and not the main site. I haven't tested it enough, but it seems to work. I will keep testing on my current project.

[QWp6t]

Instead of unnecessary nesting, the statements can be combined.

Something like this:

if (substr($url, -3) !== '/wp' && (is_main_site() || is_subdomain_install())) {

[JulienMelissas]

Thanks for this! I'll be honest in that I don't have a subdirectory multisite install I can test so I'm going to have to take your word for it 😄

Does it work everywhere else?

[greatislander]

Tested this on a subdirectory install and it works great!