LibThreading: Do not crash when detaching from an already-exited thread

Contrary to the log message that was shown, it is not decidedly a logic
error to detach a thread that has exited. Even in the simple unit tests
we have, it's possibly for the thread to run and exit in between the
time it takes for our call to Thread::start() to complete and
Thread::detach() to run. This is often seen on CI.

Userland/Libraries/LibThreading/Thread.cpp

@@ -123,11 +123,11 @@ void Thread::detach()
     auto expected = Threading::ThreadState::Running;
     // This code might race with the other thread exiting.
     if (!m_state.compare_exchange_strong(expected, Threading::ThreadState::Detached)) {
-        // Always report a precise error before crashing. These kinds of bugs are hard to reproduce.
         if (expected == Threading::ThreadState::Exited)
-            dbgln("Thread logic bug: {} is being detached after having exited!", this);
-        else
-            dbgln("Thread logic bug: trying to detach {} which is not in the Started state, but state {}!", this, expected);
+            return;
+
+        // Always report a precise error before crashing. These kinds of bugs are hard to reproduce.
+        dbgln("Thread logic bug: trying to detach {} in state {}, which is neither Started nor Exited", this, expected);
         VERIFY_NOT_REACHED();
     }