wrong "domain" returned by wpmu_validate_blog_signup when using on subsite in subdomain multisite

[maniu]

wpmu_validate_blog_signup function relies on global $domain which will give wrong results when its triggered on subsite of subdomain multisite. In cases like this, $domain will be set to current subsite, which will lead to unexpected value of "domain" element in returned array.

Triggering the function on subsite will lead to "domain" element having value like "newsite.currentsubsite.network" instead of "newsite.network".

To fix this, $domain should be set based on $current_network->domain instead of global $domain.

Changes wpmu_validate_blog_signup so it uses $domain from $current_network instead of global $domain

Trac ticket: https://core.trac.wordpress.org/ticket/58668