BUGFIX: EngMaskBitBlt() shouldn't BSOD if Mask is NULL

reactos/subsys/win32k/eng

diff -u -r1.64 -r1.65
--- bitblt.c	14 Dec 2004 03:36:46 -0000	1.64
+++ bitblt.c	18 Dec 2004 17:36:13 -0000	1.65
@@ -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: bitblt.c,v 1.64 2004/12/14 03:36:46 royce Exp $

+/* $Id: bitblt.c,v 1.65 2004/12/18 17:36:13 royce Exp $

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

@@ -901,6 +901,9 @@

   ULONG              Direction;
   POINTL             AdjustedBrushOrigin;
 

+  if ( NULL == Mask )
+    return FALSE;
+

   if (NULL != SourcePoint)
     {
     InputRect.left = SourcePoint->x;