Fix an edge case in our debug cache usage / performance reporting.
If the membuffer cache has not been used much, not all groups may
have been initialized, yet.

* subversion/libsvn_subr/cache-membuffer.c
  (svn_membuffer_get_segment_info): Exclude groups from reporting that
                                    have not been initialized, yet.
                                    They are unused by definition.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1580624 13f79535-47bb-0310-9956-ffa450edef68