In `bp_register_template_stack()` wrap `function_exists()` check with `is_array()` check. Allows class methods to be used as a callback for adding new template stack locations. Fixes #5671.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8461 cdf35c40-ae34-48e0-9cc9-0c9da1808c22