param.BigRational: Fix signum (sign) computation for positive and negative infinity

git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@12193 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/param/BigRational.java

@@ -455,6 +455,8 @@ public final class BigRational implements Comparable<BigRational>
 	 */
 	public int signum()
 	{
+		if (isInf()) return 1;
+		if (isMInf()) return -1;
 		return num.signum() * den.signum();
 	}