kernel: Do not allow sending signals to dying threads

This was causing a kernel panic when closing the terminal by directly exiting the shell (either EOF or the exit command)

kernel/src/thread/Thread.cpp

@@ -207,6 +207,7 @@ bool Thread::will_ignore_pending_signal()
 void Thread::send_signal(int signo)
 {
     if (is_kernel) return;
+    if (state == ThreadState::Exited || state == ThreadState::Dying) return;
 
     check(signo > 0 && signo <= NSIG);
     pending_signals.set(signo - 1, true);