Cygwin: pthread: Fix a race issue introduced by the commit 2c5433e5da82

Cygwin: pthread: Fix a race issue introduced by the commit 2c5433e5da82

To avoid race issues, pthread::once() uses pthread_mutex. This caused
the handle leak which was fixed by the commit 2c5433e5da82. However,
this fix introduced another race issue, i.e., the mutex may be used
after it is destroyed. This patch fixes the issue. Special thanks to
Bruno Haible for discussing how to fix this.

Addresses: https://cygwin.com/pipermail/cygwin/2024-May/255987.html
Reported-by: Bruno Haible <bruno@clisp.org>
Fixes: 2c5433e5da82 ("Cygwin: pthread: Fix handle leak in pthread_once.")
Reviewed-by: Ken Brown <kbrown@cornell.edu>
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>