In {{{bp_is_group()}}}, make sure Groups component is active before trying to get current group.

In case the groups component has been deactivated, as the WordPress page still exists, there is a possibility the Groups page is displayed. In that case, this commit prevents an error.

Fixes #5871

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9002 cdf35c40-ae34-48e0-9cc9-0c9da1808c22