make sure that the stack page is in place before disabling interrupts.

git-svn-id: file:///srv/svn/repos/haiku/trunk/current@2125 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/kernel/core/arch/x86/arch_thread.c

@@ -189,6 +189,11 @@ arch_thread_enter_uspace(addr entry, void *args, addr ustack_top)
 
 	// make sure the fpu is in a good state
 	asm("fninit");
+	
+	// access the new stack to make sure the memory page is present
+	// while interrupts are disabled.
+	// XXX does this belong there, should caller take care of it?
+	*(uint32 *)(ustack_top - 8) = 0;
 
 	disable_interrupts();