suspend-node: check if the node still exists before suspending it

After a timeout, it's possible that something else may have
destroyed the node already. This can happen for example with virtual
sinks that were created from another process.

Fixes: #467

src/scripts/suspend-node.lua

@@ -38,8 +38,11 @@ om:connect("object-added", function (om, node)
       -- add idle timeout; multiply by 1000, timeout_add() expects ms
       sources[id] = Core.timeout_add(timeout * 1000, function()
         -- Suspend the node
-        Log.info(node, "was idle for a while; suspending ...")
-        node:send_command("Suspend")
+        -- but check first if the node still exists
+        if (node:get_active_features() & Feature.Proxy.BOUND) ~= 0 then
+          Log.info(node, "was idle for a while; suspending ...")
+          node:send_command("Suspend")
+        end
 
         -- Unref the source
         sources[id] = nil