fix: fixes FATAL SIGNAL 11 on gossmap node

This will fix a crash that I caused on armv7
and by looking inside the coredump with gdb
(by adding an assert on n that must be
different from null) I get the following stacktrace

    at plugins/libplugin.c:1208

I do not know if this is a solution because I do not know
when I can parse a node announcement for a node that
it is not longer in the gossip map.

So, I hope this is just usefult for @rustyrussell

Changelog-Fixes: fixes `FATAL SIGNAL 11` on gossmap node announcement parsing.

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

common/gossmap.c

@@ -582,8 +582,8 @@ static void node_announcement(struct gossmap *map, size_t nann_off)
 
 	feature_len = map_be16(map, nann_off + feature_len_off);
 	map_nodeid(map, nann_off + feature_len_off + 2 + feature_len + 4, &id);
-	n = gossmap_find_node(map, &id);
-	n->nann_off = nann_off;
+	if ((n = gossmap_find_node(map, &id)))
+		n->nann_off = nann_off;
 }
 
 static void reopen_store(struct gossmap *map, size_t ended_off)