prevent `lda @y` or `lda @x` (which is accepted by the parser) from crashing.

expressionSemantics.c

@@ -1109,7 +1109,8 @@ evaluateExpressionInternally(expressionType *expression, bool isTopLevel, fixupK
 evaluateExpression(expressionType *expression, fixupKindType kindOfFixup)
 {
 	valueType	*result;
-
+	nullEvaluate(expression);
+	
 	expressionFailed = FALSE;
 	referencesToNote[currentOperandNumber] = NULL;
 	result = evaluateExpressionInternally(expression, TRUE, kindOfFixup,

operandStuffSD_6502.c

@@ -299,6 +299,7 @@ evaluateOperand(operandType *operand)
 	case Y_INDEXED_OPND:
 		result = evaluateExpression(operand->theOperand.expressionUnion,
 			performingFixups ? NO_FIXUP : OPERAND_FIXUP);
+		nullEvaluate(result);
 		if (operand->kindOfOperand != EXPRESSION_OPND) {
 			if (result->addressMode != EXPRESSION_OPND) {
 				error(BAD_ADDRESS_MODE_ERROR);