AP_Bootloader: reject allocation of broadcast node ID

It is technically legal to receive an "allocation" of the broadcast node
ID. Fortunately, this was already ignored by `canardSetLocalNodeID`,
though it would trigger an assertion failure if those were enabled.

Fix by rejecting that ID. There is effectively no change in behavior but
the code now correctly ignores that ID and retries the allocation as it
did before.

Tools/AP_Bootloader/can.cpp

@@ -429,7 +429,7 @@ static void handle_allocation_response(CanardInstance* ins, CanardRxTransfer* tr
         // The allocator has confirmed part of unique ID, switching to the next stage and updating the timeout.
         node_id_allocation_unique_id_offset = msg.unique_id.len;
         send_next_node_id_allocation_request_at_ms -= UAVCAN_PROTOCOL_DYNAMIC_NODE_ID_ALLOCATION_MIN_REQUEST_PERIOD_MS;
-    } else {
+    } else if (msg.node_id != CANARD_BROADCAST_NODE_ID) { // new ID valid? (if not we will time out and start over)
         // Allocation complete - copying the allocated node ID from the message
         canardSetLocalNodeID(ins, msg.node_id);
     }