BUG/MINOR: listener: close tiny race between resume_listener() and stopping

Pierre Cheynier reported a very rare race condition on soft-stop in the
listeners. What happens is that if a previously limited listener is
being resumed by another thread finishing an accept loop, and at the
same time a soft-stop is performed, the soft-stop will turn the
listener's state to LI_INIT, and once the listener's lock is released,
resume_listener() in the second thread will try to resume this listener
which has an fd==-1, yielding a crash in listener_set_state():

  FATAL: bug condition "l->rx.fd == -1" matched at src/listener.c:288

The reason is that resume_listener() only checks for LI_READY, but doesn't
consider being called with a non-initialized or a stopped listener. Let's
also make sure we don't try to ressuscitate such a listener there.

This will have to be backported to all versions.

src/listener.c

@@ -530,6 +530,10 @@ int resume_listener(struct listener *l, int lpx)
 	if (l->state == LI_READY)
 		goto end;
 
+	/* the listener might have been stopped in parallel */
+	if (l->state < LI_PAUSED)
+		goto end;
+
 	if (l->rx.proto->resume)
 		ret = l->rx.proto->resume(l);