Fix compiler warning about sign conversion when compare the len variable
to the actual length by subtracting pointers.

* subversion/libsvn_subr/x509.h
  (x509_buf): Switch len member from apr_size_t to ptrdiff_t.

* subversion/libsvn_subr/x509parse.c
  (asn1_get_len, asn1_get_tag, asn1_get_int, x509_get_version, x509_get_alg,
   x509_get_name, x509_get_date, x509_get_dates, x509_get_sig, x509_get_ext,
   svn_x509_parse_cert): Change length variables to use ptrdiff_t instead of
    apr_size_t.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/svn-auth-x509@1616630 13f79535-47bb-0310-9956-ffa450edef68