Fix condition for assuming at EOF

Part of that condition's purpose is to ensure that we read the correct
lexer state; but it's possible now for the fstack to be non-empty
*before* the lexer state is registered, i.e. if there is an error
in the function that registers it.
This causes a NULL pointer deref.

include/asm/lexer.hpp

@@ -131,6 +131,7 @@ static inline void lexer_SetGfxDigits(char const digits[4]) {
 	gfxDigits[3] = digits[3];
 }
 
+bool lexer_AtTopLevel();
 void lexer_RestartRept(uint32_t lineNo);
 void lexer_Init();
 void lexer_SetMode(LexerMode mode);