Fix page fault on accessing initial thread frame

xtoskrnl/ke/i686/kthread.c

@@ -43,7 +43,7 @@ KepInitializeThreadContext(IN PKTHREAD Thread,
     PFX_SAVE_FORMAT FxSaveFormat;
 
     /* Set initial thread frame */
-    ThreadFrame = ((PKTHREAD_INIT_FRAME)Thread->InitialStack) - sizeof(KTHREAD_INIT_FRAME);
+    ThreadFrame = (PKTHREAD_INIT_FRAME)(Thread->InitialStack - sizeof(KTHREAD_INIT_FRAME));
 
     /* Fill floating point save area with zeroes */
     RtlZeroMemory(&ThreadFrame->NpxFrame, sizeof(FX_SAVE_AREA));