alsa.lua: remove node names from table when parent device is destroyed

Fixes #293

src/scripts/monitors/alsa.lua

@@ -345,6 +345,9 @@ function createMonitor ()
       end
     end
     device_names_table[device.properties["device.name"]] = nil
+    for managed_node in device:iterate_managed_objects() do
+      node_names_table[managed_node.properties["node.name"]] = nil
+    end
   end)
 
   -- reset the name tables to make sure names are recycled