set proper lasterror code in RegisterWindowMessage()

reactos/subsys/win32k/ntuser

diff -u -r1.234 -r1.235
--- window.c	19 May 2004 19:09:20 -0000	1.234
+++ window.c	19 May 2004 19:16:47 -0000	1.235
@@ -16,7 +16,7 @@

  *  along with this program; if not, write to the Free Software
  *  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */

-/* $Id: window.c,v 1.234 2004/05/19 19:09:20 weiden Exp $

+/* $Id: window.c,v 1.235 2004/05/19 19:16:47 weiden Exp $

  *
  * COPYRIGHT:        See COPYING in the top level directory
  * PROJECT:          ReactOS kernel

@@ -3576,6 +3576,12 @@

   NTSTATUS Status;
   UINT Ret;
   

+  if(MessageNameUnsafe == NULL)
+  {
+    SetLastWin32Error(ERROR_INVALID_PARAMETER);
+    return 0;
+  }
+  

   Status = IntSafeCopyUnicodeStringTerminateNULL(&SafeMessageName, MessageNameUnsafe);
   if(!NT_SUCCESS(Status))
   {