Multisite: Ensure that get_blogaddress_by_name does not mangle blognames with leading digits.

Correctly specify the backreference in the regular expression so that it can not become ambiguous when there is a leading digit on the blogname.

Fixes #23689 props dllh.


git-svn-id: https://core.svn.wordpress.org/trunk@23686 1a063a9b-81f0-0310-95a4-ce76da25c4cd