Fix signed/unsigned mismatch warning.

* subversion/libsvn_subr/checksum.c
  (svn__digest_to_cstring_display): Use apr_size_t instead int for index.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1577070 13f79535-47bb-0310-9956-ffa450edef68