XProfile: Check for activity component before using bp_activity_get().

Currently, the bp_xprofile_updated_profile_activity() function does not
check if the activity component is active before using bp_activity_get().

This can potentially cause a fatal error when updating one's profile.

Fixes #5571 (trunk)

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8306 cdf35c40-ae34-48e0-9cc9-0c9da1808c22