Make the use of memcmp safe for fs_fs__id_t.  For that, all bytes of
the struct must have a well-defined contents.  The structs must to
be allocated by apr_pcalloc or duplicated bytewise.

* subversion/libsvn_fs_fs/id.c
  (svn_fs_fs__id_copy): To copy IDs, use APR's duplication function
                        instead of non-zeroing allocation + assignment.

Found by: kotkov                        

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1619358 13f79535-47bb-0310-9956-ffa450edef68