buckets: Don't crash on request for keys before init

This situation is rare, but actually can happen, because the client may
reconnect/restart or join the group before it received the config. Just
warn in such case instead of crashing and ignore the request (the server
will be OK if it doesn't get the answer, it waits for a timeout anyway).

src/plugins/buckets/buckets.c

@@ -438,7 +438,11 @@ static void communicate(struct context *context, const uint8_t *data, size_t len
 			}
 			return;
 		case 'K': // Send keys
-			sanity(context->user_data->initialized, "Asked to send keys before initialization\n"); // The server should track who it asks
+			if (!context->user_data->initialized) {
+				// This can happen on reconnect, for example, when we lost the config.
+				ulog(LLOG_WARN, "Asked to send bucket keys before initialization\n");
+				return;
+			}
 			provide_keys(context, data + 1, length - 1);
 			return;
 		default: