Fix logic error in caching in BP_Groups_Group::populate()

Introduced in r7956, this bug cause the populate logic not to be run when
the cache was hit successfully. Mega-anti-props boonebgorges.

See #5407

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7974 cdf35c40-ae34-48e0-9cc9-0c9da1808c22