Fix krb5's gss_pseudo_random() (n is big-endian)

The first enctype RFC3961 prf output length's bytes are correct because
the little- and big-endian representations of unsigned zero are the
same.  The second block of output was wrong because the counter was not
being encoded as big-endian.

This change could break applications.  But those applications would not
have been interoperating with other implementations anyways (in
particular: MIT's).