kernel: Retrieve all thread information before marking it as dead in waitpid()

In case we get interrupted by the reaper thread.

kernel/src/sys/waitpid.cpp

@@ -43,11 +43,12 @@ Result<u64> sys_waitpid(Registers*, SyscallArgs args)
         return err(ENOTSUP);
 
     int status = (int)thread->status;
+    u64 id = thread->id;
 
     thread->state = ThreadState::Dying;
 
     if (status_ptr)
         if (!MemoryManager::copy_to_user_typed(status_ptr, &status)) return err(EFAULT);
 
-    return thread->id;
+    return id;
 }