On svn-auth-x509 branch, silence warnings about 64 to 32-bit conversions.

* subversion/libsvn_subr/x509parse.c
  (svn_x509_parse_cert): Cast to int when calculating lengths.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/svn-auth-x509@1605824 13f79535-47bb-0310-9956-ffa450edef68