LibCore: Avoid UAF on the array of wake pipes when exit()ing

If exit() is called on a thread with an EventLoop in the stack, the
ThreadData storing the array of wake pipes will be destroyed first.
Threads can still take a strong reference to the EventLoop after that,
and will read the fds from freed memory.

Instead, take a copy of the write fd, and swallow EBADF when writing to
it, since that only indicates that the thread and event loop are
exiting, so there's nothing to do with the wake.

Libraries/LibCore/EventLoopImplementationUnix.h

@@ -55,8 +55,9 @@ private:
     bool m_exit_requested { false };
     int m_exit_code { 0 };
 
-    // The wake pipe of this event loop needs to be accessible from other threads.
-    Array<int, 2>& m_wake_pipe_fds;
+    // The write end of the wake pipe, copied by value so it remains valid even
+    // if ThreadData is destroyed before this event loop (e.g. during exit()).
+    int m_wake_pipe_write_fd;
 };
 
 using EventLoopManagerPlatform = EventLoopManagerUnix;