Use emergency TEB selector to avoid debugger crashes when stepping
through relay stubs (or when FS is invalid otherwise).

diff --git a/include/syslevel.h b/include/syslevel.h
index e151075..d2756dc 100644
--- a/include/syslevel.h
+++ b/include/syslevel.h
@@ -11,6 +11,7 @@
 #include "winbase.h"
 
 extern WORD SYSLEVEL_Win16CurrentTeb;
+extern WORD SYSLEVEL_EmergencyTeb;
 
 void SYSLEVEL_Init(void);
 VOID SYSLEVEL_EnterWin16Lock(VOID);