Cygwin: thread: Reset _my_tls.tid if it's pthread_null in init_mainthread().

Currently, _my_tls.tid is set to pthread_null if pthread::self()
is called before pthread::init_mainthread(). As a result, pthread::
init_mainthread() does not set _my_tls.tid appropriately. Due to
this, pthread_join() fails in LDAP environment if the program is
the first program which loads cygwin1.dll.

https://cygwin.com/pipermail/cygwin/2023-June/253792.html

With this patch, _my_tls.tid is re-initialized in pthread::
init_mainthread() if it is pthread_null.

Reported-by: Mümin A. <muminaydin06@gmail.com>
Reviewed-by: Corinna Vinschen <corinna@vinschen.de>
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>

winsup/cygwin/thread.cc

@@ -364,7 +364,7 @@ void
 pthread::init_mainthread ()
 {
   pthread *thread = _my_tls.tid;
-  if (!thread)
+  if (!thread || thread == pthread_null::get_null_pthread ())
     {
       thread = new pthread ();
       if (!thread)