Fix common rtorrent xml-rpc crash when trying to queue an invalid task

Instead of throwing an internal error and terminating the client, it's better not to queue the invalid task in the first place.
`C Caught internal_error: 'priority_queue_insert(...) called on an invalid item.'.`

src/rpc/command_scheduler_item.cc

@@ -53,10 +53,14 @@ CommandSchedulerItem::enable(rak::timer t) {
 
   if (is_queued())
     disable();
+
+  // Don't schedule invalid tasks for rpc commands
+  if (!m_task.is_valid())
+    return;
 
   // If 'first' is zero then we execute the task
   // immediately. ''interval()'' will not return zero so we never end
-  // up in an infinit loop.
+  // up in an infinite loop.
   m_timeScheduled = t;
   priority_queue_insert(&taskScheduler, &m_task, t);
 }