accumulation: fix behaviour for step 0

prism/src/explicit/AccumulationProductCounting.java

@@ -153,8 +153,11 @@ public class AccumulationProductCounting<M extends Model> extends AccumulationPr
 			throws PrismException {
 		boolean isFinal = false;
 		if ( track != null ) {
-			IntegerBound stepBound = IntegerBound.fromTemporalOperatorBound(accexp.getBoundExpression(), mc.getConstantValues(), true);
-			isFinal = stepBound.isInBounds(track.getComponent());
+			Integer step = track.getComponent();
+			if ( step > 0 ) { // if the step is 0, we cannot have a goal state.
+				IntegerBound stepBound = IntegerBound.fromTemporalOperatorBound(accexp.getBoundExpression(), mc.getConstantValues(), true);
+				isFinal = stepBound.isInBounds(step);
+			}
 		}
 		return isFinal;
 	}