cleanup: get rid of httpdigest.c type warnings

cleanup: get rid of httpdigest.c type warnings

When I compile with --enable-strict, I only want to see warnings that are
relevant.  So, change httpdigest.c to make the casts explicit.

This commit should not change behaviour.

v2: as discussed on #openvpn-devel, make colon a const uint8_t *, instead
    of uint8_t.
v3: as further discussed on #openvpn-devel, don't use a 'colon' var, but
    just add casts.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1450646868-15346-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10871
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 0385cd4804c133d48857e4b3fbfe93a75ecc68a5)