Incorrect handling of parametric sink-adjacent arcs.

[quic0]

The following instance has a a breakpoint at 1.5. HPF doesn't find a breakpoint.

    G = DiGraph()
    G.add_nodes_from(['s', 't', 1, 2])
    G.add_edges_from([('s', 1), (1, 't'), (1, 2), (2, 't')])

    G['s'][1]["const"] = 2
    G[1][2]["const"] = 5
    G[1]['t']["const"] = 0
    G[2]['t']["const"] = float('inf')

    G['s'][1]["mult"] = 1
    G[1]['t']["mult"] = -1
    G[1][2]["mult"] = 0
    G[2]['t']["mult"] = 0

[quic0]

Issue: arc (1,t) has negative capacity.
We could increase the constant on (1, t) to 3 and reduce constant on (1, 2) to 2.