Convert total_friend_count to SIGNED before attempting to sort with it

This fixes issues in BP_User_Query where the 'popular' sort order was sorting
by friend count interpreted as a string (ie, alphabetically) instead of
numerically.

Fixes #4937

Props merty

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6996 cdf35c40-ae34-48e0-9cc9-0c9da1808c22