get_route: fix path_min for first_hop<>network_node candidates

get_route: fix path_min for first_hop<>network_node candidates

Previously, we would add a first_hop<>network_node channel that did not have
enough contribution amount to cover the next channel's min htlc plus fees,
because we were storing the next hop as having a path_min that did not include
fees, and would add a connecting first_hop node that did not have enough
contribution amount, leading to a debug panic upon invalid path construction.