MINOR: listener: add a few BUG_ON() statements to detect inconsistencies

We must not have an fd==-1 when switching to certain states. This will
later disappear but for now it helps detecting inconsistencies.
diff --git a/src/listener.c b/src/listener.c
index 6368e37..48fdc96 100644
--- a/src/listener.c
+++ b/src/listener.c
@@ -255,14 +255,17 @@
 		case LI_ASSIGNED:
 			break;
 		case LI_PAUSED:
+			BUG_ON(l->rx.fd == -1);
 			_HA_ATOMIC_ADD(&px->li_paused, 1);
 			break;
 		case LI_LISTEN:
+			BUG_ON(l->rx.fd == -1);
 			_HA_ATOMIC_ADD(&px->li_bound, 1);
 			break;
 		case LI_READY:
 		case LI_FULL:
 		case LI_LIMITED:
+			BUG_ON(l->rx.fd == -1);
 			_HA_ATOMIC_ADD(&px->li_ready, 1);
 			break;
 		}
@@ -280,6 +283,7 @@
 {
 	HA_SPIN_LOCK(LISTENER_LOCK, &listener->lock);
 	if (listener->state == LI_LISTEN) {
+		BUG_ON(listener->rx.fd == -1);
 		if ((global.mode & (MODE_DAEMON | MODE_MWORKER)) &&
 		    !(proc_mask(listener->rx.settings->bind_proc) & pid_bit)) {
 			/* we don't want to enable this listener and don't