peer: fix close before their sig has been received in corner case.

If we haven't received their closing signature yet, we might try to
send the closing packet anyway (and segfault).  Make sure we have
their signature before trying that.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

daemon/peer.c

@@ -1382,7 +1382,7 @@ static bool shutdown_pkt_in(struct peer *peer, const Pkt *pkt)
 		break;
 	}
 
-	if (!err && !committed_to_htlcs(peer))
+	if (!err && !committed_to_htlcs(peer) && peer->closing.their_script)
 		err = start_closing(peer);
 
 	if (err)