ExplicitFiles2MTBDD: fix missing JDDNode copy

When using the init label DD for the actual start state, need to copy.
Otherwise the ref count will be too low.


git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@11996 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/prism/ExplicitFiles2MTBDD.java

@@ -673,7 +673,7 @@ public class ExplicitFiles2MTBDD
 		}
 		// Otherwise, construct from the labels
 		else {
-			start = labelsDD.get("init");
+			start = labelsDD.get("init").copy();
 			if (start == null || start.equals(JDD.ZERO)) {
 				throw new PrismException("No initial states found in labels file");
 			}