Cygwin: posix timers: fix resource leak

On setting the timer, the thread is accidentally only canceled when
disarming the timer.  This leaks one thread per timer_settimer call.
Move the thread cancellation where it belongs.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/posix_timer.cc

@@ -287,9 +287,9 @@ timer_tracker::settime (int flags, const itimerspec *new_value,
       if (old_value)
 	gettime (old_value, false);
 
+      cancel ();
       if (!new_value->it_value.tv_sec && !new_value->it_value.tv_nsec)
 	{
-	  cancel ();
 	  memset (&time_spec, 0, sizeof time_spec);
 	  interval = 0;
 	  exp_ts = 0;