[core] Print critical warning if process tree is broken

Log critical warning each time rebuild_tree() is invoked, as such
errors should be closely examined. Maybe I have missed something, but
couldn't found out how this may happen.

src/core.c

@@ -1283,6 +1283,9 @@ static void rebuild_tree() {
   gpointer key, value;
   u_proc *proc, *parent;
 
+  // could this ever happen?
+  g_critical("Process tree is desynchronized, rebuilding.");
+
   // clear root node
   g_node_destroy(processes_tree);
   processes_tree = g_node_new(NULL);