Blogs: Add check for activity component when recording a post comment.

Previously, if the activity component was disabled and the blogs component
was enabled, if a user posted a blog comment, a WSOD would occur.

This commit fixes this by adding a check for the activity component in
the bp_blogs_record_comment() function.

See #5606 (trunk)

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8343 cdf35c40-ae34-48e0-9cc9-0c9da1808c22