some case from win32k can call to RtlClearAllBits with NULL pointer. and check for null pointer after RtlClearAllBits. This take care of those case for moment.

Modified: trunk/reactos/lib/rtl/bitmap.c

--- trunk/reactos/lib/rtl/bitmap.c	2005-12-29 13:13:14 UTC (rev 20420)
+++ trunk/reactos/lib/rtl/bitmap.c	2005-12-29 13:43:25 UTC (rev 20421)
@@ -125,9 +125,15 @@

 VOID NTAPI
 RtlClearAllBits(IN OUT PRTL_BITMAP BitMapHeader)
 {

-    memset(BitMapHeader->Buffer,
-	   0x00,
-	   ROUND_UP(BitMapHeader->SizeOfBitMap, 8) / 8);

+   if (BitMapHeader != NULL)
+   { 
+     if (BitMapHeader->Buffer != NULL)
+     {
+        memset(BitMapHeader->Buffer,
+	           0x00,
+	           ROUND_UP(BitMapHeader->SizeOfBitMap, 8) / 8);
+     }
+   }

 }