batman-adv: fix gw_node_update() and gw_election()

This is a regression from c4aac1ab

- gw_node_update() doesn't add a new gw_node in case of empty curr_gw.
This means that at the beginning no gw_node is added, leading to an
empty gateway list.

- gw_election() is terminating in case of curr_gw == NULL. It has to
terminate in case of curr_gw != NULL

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Sven Eckelmann <sven@narfation.org>