lightningd/htlc_end: fix key for stricter compiler.

We *should* split the struct into key and data, rather than only comparing
the key parts in the htlc_end_eq function.  But meanwhile, this fixes
the code.

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

lightningd/htlc_end.c

@@ -22,7 +22,7 @@ struct htlc_end *find_htlc_end(const struct htlc_end_map *map,
 			       enum htlc_end_type which_end)
 {
 	const struct htlc_end key = { which_end, (struct peer *)peer, htlc_id,
-				      NULL };
+				      0, NULL, NULL };
 
 	return htlc_end_map_get(map, &key);
 }