On svn-auth-x509 branch, cast to avoid sign mistmatch warning.

* subversion/libsvn_subr/x509parse.c:
  (svn_x509_parse_cert): Cast buf to unsigned before doing arithmetic with it.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/svn-auth-x509@1605804 13f79535-47bb-0310-9956-ffa450edef68