gossipd: fix crash if we truncate store.

Entries we've already loaded expect to exist in the store.  We could go
back and remove them all, but instead just truncate at the known-good
point.

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

gossipd/gossip_store.c

@@ -550,13 +550,13 @@ void gossip_store_load(struct routing_state *rstate, struct gossip_store *gs)
 
 truncate:
 	status_unusual("gossip_store: %s (%s) truncating to %"PRIu64,
-		       bad, tal_hex(msg, msg), (u64)1);
+		       bad, tal_hex(msg, msg), gs->len);
 truncate_nomsg:
 	/* FIXME: We would like to truncate to known_good, except we would
 	 * miss channel_delete msgs.  If we put block numbers into the store
 	 * as we process them, we can know how far we need to roll back if we
 	 * truncate the store */
-	if (ftruncate(gs->fd, 1) != 0)
+	if (ftruncate(gs->fd, gs->len) != 0)
 		status_failed(STATUS_FAIL_INTERNAL_ERROR,
 			      "Truncating store: %s", strerror(errno));
 out: