Cygwin: pty: Fix crash on master close in Windows 7.

- The 4th parameter of WriteFile() cannot be NULL especially in
  Windows 7 as mentioned in Microsoft documentation. This patch
  fixes that.

Addresses: https://cygwin.com/pipermail/cygwin/2022-March/251162.html

winsup/cygwin/fhandler_tty.cc

@@ -2172,7 +2172,7 @@ fhandler_pty_master::close ()
 	    }
 	  release_output_mutex ();
 	  get_ttyp ()->stop_fwd_thread = true;
-	  WriteFile (to_master_nat, "", 0, NULL, NULL);
+	  WriteFile (to_master_nat, "", 0, &len, NULL);
 	  master_fwd_thread->detach ();
 	}
     }

winsup/cygwin/release/3.3.5

@@ -43,3 +43,6 @@ Bug Fixes
 
 - Fix a formatting problem in gmondump where all displayed addresses are
   mistakenly prefixed with "0x0x".
+
+- Fix crash on pty master close in Windows 7.
+  Addresses: https://cygwin.com/pipermail/cygwin/2022-March/251162.html