param/exact: Fix Pmax/Pmin [ ! (F a) ] / [ !( a U b) ] computation

Switching between max and min when removing the negation was missing for MDPs.

Test case:
prism functionality/verify/mdps/ltl/simple_ltl.nm functionality/verify/mdps/ltl/simple_ltl.nm.props -exact -prop 3
from prism-tests.

prism/src/param/ParamModelChecker.java

@@ -903,6 +903,7 @@ final public class ParamModelChecker extends PrismComponent
 		if (expr instanceof ExpressionUnaryOp &&
 		    ((ExpressionUnaryOp)expr).getOperator() == ExpressionUnaryOp.NOT) {
 			negated = true;
+			min = !min;
 			expr = ((ExpressionUnaryOp)expr).getOperand();
 		}