Cygwin: signal: Drop unnecessary queue flush

Previously, the retry flag was always set when pending_signal::pending()
was called. However, if the queue is empty sig thread tries to flush
the queue even though it is not necessary. With this patch, the retry
flag is set only if the queue is not empty.

Addresses: https://cygwin.com/pipermail/cygwin/2024-November/256744.html
Fixes: 5e31c80e4e ("(pending_signals::pending): Force an additional loop through wait_sig by setting retry whenever this function is called.")
Reported-by: Christian Franke <Christian.Franke@t-online.de>
Reviewed-by: Corinna Vinschen <corinna@vinschen.de>
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>

winsup/cygwin/sigproc.cc

@@ -110,7 +110,7 @@ class pending_signals
 
 public:
   void add (sigpacket&);
-  bool pending () {retry = true; return !!start.next;}
+  bool pending () {retry = !!start.next; return retry;}
   void clear (int sig) {sigs[sig].si.si_signo = 0;}
   void clear (_cygtls *tls);
   friend void sig_dispatch_pending (bool);