Blogs: Check if activity component is active before using activity meta functions.

Previously, r8944 used some functions to mirror the blog post title to the
activity meta table.  However, a check was not made to determine if the
activity component was active.  This caused a fatal error for sites that
have the activity component disabled.

Fixes #5897 (trunk).

Anti-props r-a-y.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9039 cdf35c40-ae34-48e0-9cc9-0c9da1808c22