odhcpd: fix captive_portal_uri reset

odhcpd: fix captive_portal_uri reset

odhcpd_reload() calls clean_interface() which does a memset() on the
given struct iface from iface->ra onwards, so captive_portal_uri and
captive_portal_uri_len have to be below iface->ra in order to be zeroed
on reload.

Otherwise, iface->captive_portal_uri will be free():d, but
iface->captive_portal_uri_len could still be > 0.

This is a bit of a footgun (one wouldn't normally expect the order of
struct members to matter), and should probably be dealt with in a more
thorough manner, but I've just done the minimum and added a warning in
src/odhcpd.h for now,

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/345
Signed-off-by: Álvaro Fernández Rojas <[email protected]>