Kernel: Mark the idle thread as active before switching it in

Otherwise, our code for dumping all thread stacks gets confused if it
finds a thread that is in a 'running' state, but that isn't marked
active.

Kernel/Scheduler.cpp

@@ -99,7 +99,10 @@ Thread& Scheduler::pull_next_runnable_thread()
             }
             priority_mask &= ~(1u << priority);
         }
-        return *Processor::idle_thread();
+
+        auto* idle_thread = Processor::idle_thread();
+        idle_thread->set_active(true);
+        return *idle_thread;
     });
 }