batman-adv: Fix check of retrieved orig_gw in batadv_v_gw_is_eligible

The batadv_v_gw_is_eligible function already assumes that orig_node is not
NULL. But batadv_gw_node_get may have failed to find the originator. It
must therefore be checked whether the batadv_gw_node_get failed and not
whether orig_node is NULL to detect this error.

Fixes: 50164d8f ("batman-adv: B.A.T.M.A.N. V - implement GW selection logic")
Signed-off-by: Sven Eckelmann <sven.eckelmann@openmesh.com>
Acked-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>