coauthor_slug is sometimes based on user_login rather than user_nicename

I don't know how it happened, but several months' worth of posts (in the July-September 2012 range) appear to have a coauthor_slug set to cap-{user_login} instead of cap-{user_nicename}, which causes all of the coauthors information (authors column, edit page metabox, coauthors() function) to be empty.  I noticed the empty fields after upgrading to WordPress 3.5.

This commit tries to find the slug in user_login if it's not found in user_nicename.