net: rpl: fix null pointer dereference

This patch fixes possible null pointer dereference in
net_stats_update_rpl_resets(...).

net_rpl_set_root_with_version(...) does not initialize instance->iface
and calls net_rpl_reset_dio_timer(...), which then calls
net_stats_update_rpl_resets(instance->iface).

fixes: #7862

Signed-off-by: Johann Fischer <j.fischer@phytec.de>