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.

src/asm/lexer.cpp

@@ -338,6 +338,10 @@ static bool isWhitespace(int c) {
 static LexerState *lexerState = nullptr;
 static LexerState *lexerStateEOL = nullptr;
 
+bool lexer_AtTopLevel() {
+	return lexerState == nullptr;
+}
+
 void LexerState::clear(uint32_t lineNo_) {
 	mode = LEXER_NORMAL;
 	atLineStart = true; // yylex() will init colNo due to this