Fix evaluateExact for unary minus

We should propagate the EvaluateContext into the inner expression.

The missing 'ec' led to exceptions in model construction for exact/parametric mode, as variable valuations are then not available, e.g., when used in guards ('-s' where 's' is a state variable).

Checked the other evaluateExact calls, those are fine.

prism/src/parser/ast/ExpressionUnaryOp.java

@@ -144,7 +144,7 @@ public class ExpressionUnaryOp extends Expression
 		case NOT:
 			return BigRational.from(!operand.evaluateExact(ec).toBoolean());
 		case MINUS:
-			return operand.evaluateExact().negate();
+			return operand.evaluateExact(ec).negate();
 		case PARENTH:
 			return operand.evaluateExact(ec);
 		}