Cygwin: sigpending: don't report pending signals for other threads

The sigpending mechanism failed to check if the pending signal was a
process-wide signal, or a signal for the curent thread.  Fix that by
adding a matching conditional to wait_sig's __SIGPENDING code.

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

winsup/cygwin/release/3.1.0

@@ -37,3 +37,6 @@ Bug Fixes
 
 - Fix a timerfd deadlock.
   Addresses: https://cygwin.com/ml/cygwin/2019-06/msg00096.html
+
+- Fix sigpending() incorrectly returning signals for unrelated threads.
+  Addresses: https://cygwin.com/ml/cygwin/2019-07/msg00051.html

winsup/cygwin/sigproc.cc

@@ -1335,8 +1335,13 @@ wait_sig (VOID *)
 	    *pack.mask = 0;
 	    tl_entry = cygheap->find_tls (pack.sigtls);
 	    while ((q = q->next))
-	      if (pack.sigtls->sigmask & (bit = SIGTOMASK (q->si.si_signo)))
-		*pack.mask |= bit;
+	      {
+		/* Skip thread-specific signals for other threads. */
+		if (q->sigtls && pack.sigtls != q->sigtls)
+		  continue;
+		if (pack.sigtls->sigmask & (bit = SIGTOMASK (q->si.si_signo)))
+		  *pack.mask |= bit;
+	      }
 	    cygheap->unlock_tls (tl_entry);
 	  }
 	  break;