gossip_store: handle compacted gossip_store correctly.

Don't send EOF marker to peer, e.g. in tests/test_gossip.py::test_gossip_store_compact:

```
lightningd-2: 2022-01-24T03:34:22.925Z DEBUG   connectd: gossip_store at end, new fd moved to 1875
lightningd-2: 2022-01-24T03:34:22.933Z DEBUG   035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d-connectd: Sending gossip INVALID 4105
lightningd-2: 2022-01-24T03:34:22.933Z DEBUG   035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d-channeld-chan#2: peer_in WIRE_WARNING
lightningd-2: 2022-01-24T03:34:22.941Z DEBUG   035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d-connectd: peer_out INVALID 4105
lightningd-2: 2022-01-24T03:34:22.949Z DEBUG   035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d-channeld-chan#2: billboard perm: Received warning channel 2c7cf1dc9dada7ed14f10c78ade8f0de907c1b70e736c12ff6f7472dc69c3db3: Peer sent unknown message 4105 (INVALID 4105)
```

Signed-off-by: Rusty Russell <[email protected]>

common/gossip_store.c

@@ -99,6 +99,7 @@ u8 *gossip_store_next(const tal_t *ctx,
 		/* end can go backwards in this case! */
 		if (type == WIRE_GOSSIP_STORE_ENDED) {
 			*off = *end = reopen_gossip_store(gossip_store_fd, msg);
+			msg = tal_free(msg);
 		/* Ignore gossipd internal messages. */
 		} else if (type != WIRE_CHANNEL_ANNOUNCEMENT
 			   && type != WIRE_CHANNEL_UPDATE