Pass 'per_page' argument to bp_has_members() in BP_Core_Members_Widget

This argument is needed for cases where 'max' is less than the 'per_page'
default value of 20.

Fixes #5410

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7928 cdf35c40-ae34-48e0-9cc9-0c9da1808c22