param.BigRational: fix pow() to correctly handle negative exponents

Previously, using BigRational.pow(exponent) with negative exponent would result in exception


git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@12155 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/param/BigRational.java

@@ -474,6 +474,7 @@ public final class BigRational implements Comparable<BigRational>
 			num = this.num.pow(exponent);
 			den = this.den.pow(exponent);
 		} else { // exponent < 0
+			exponent = -exponent;
 			num = this.den.pow(exponent);
 			den = this.num.pow(exponent);
 		}