(v1.x) 2x listen possible with undefined behavior, trivially avoided

[benfleis]

In v1.x code, .listen() can be called twice in a destructive way. I don't completely understand the path, but the evidence was clear. When using nodejs/cluster, 2 calls to .listen() can occur, and can create 2 different sockets, both listening on the same port. (Observed via lsof.)

This is avoided in v2 by asserting that .listen isn't already called, and I would propose adding a similar trivial patch to the v1.x branch.