In `bp_core_load_template()`, save a `locate_template()` call.

When the current theme isn't the bp-default theme (or a deriviative), we do
not need to attempt to find a bp-default template since it will not exist.

This saves us an unnecessary `locate_template()` call and a few
`file_exists()` checks.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8820 cdf35c40-ae34-48e0-9cc9-0c9da1808c22