'svnadmin info': Fix a future segfault.

* subversion/svnadmin/svnadmin.c
  (subcommand_info): Explicitly convert the array element to svn_sort__item_t.
    Using APR_ARRAY_IDX(_, _, const char *) would segfault once the array has
    two elements (because it would mis-compute the array stride).

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1483955 13f79535-47bb-0310-9956-ffa450edef68