MFC r298731, r298734:

MFC r298731, r298734:
ndis(4): unsign some indexes to prevent overflows.

The "len" parameter is uint32_t, indexing it with an int may
end up in a signed integer overflow.

strlen(3) returns an integer of size_t but a correponding index
of type u_int is more than enough.

git-svn-id: svn://svn.freebsd.org/base/stable/10@299103 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f