Bail from bp_activity_remove_user_favorite() if the user has not previously favorited the item in question

The failure to do this check could result in the incorrect decrementing of the
item's favorite_count.

Fixes #5478

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8177 cdf35c40-ae34-48e0-9cc9-0c9da1808c22