Bugfix: c-closure on empty zone breaks when max clock constraint constant is 0.

git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@5582 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/pta/DBM.java

@@ -219,6 +219,8 @@ public class DBM extends Zone
 	public void cClosure(int c)
 	{
 		int i, j, n;
+		if (isEmpty())
+			return;
 		n = pta.numClocks;
 		for (i = 0; i < n + 1; i++) {
 			for (j = 0; j < n + 1; j++) {