Members: set the displayed user's `fullname` to their display name

This was broken in r8610 because it was trying to access a property in the WP_User object that doesn't exist.

Fixes #5764, props imath

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8660 cdf35c40-ae34-48e0-9cc9-0c9da1808c22