Bug fix in PTA model checking (digital clocks): GCD of {0} is 1.

git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@3520 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/pta/DigitalClocks.java

@@ -454,6 +454,10 @@ public class DigitalClocks
 			for (int x : ints) {
 				gcd = computeGCD(gcd, x);
 			}
+			if (gcd == 0) {
+				// For the case where clock set is empty or all zeros
+				gcd = 1;
+			}
 			return gcd;
 		}