In `bp_get_activity_comment_permalink()`, check that activity comment exists, and fallback on the regular activity permalink if it doesn't. Also pass `$comment_id` into the filter for easier tweaking.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8717 cdf35c40-ae34-48e0-9cc9-0c9da1808c22