8242241: add assert to ClassUnloadEventImpl::className

Add assert

Reviewed-by: cjplummer, dcubed

src/jdk.jdi/share/classes/com/sun/tools/jdi/EventSetImpl.java

@@ -425,6 +425,7 @@ public class EventSetImpl extends ArrayList<Event> implements EventSet {
         }
 
         public String className() {
+            assert classSignature.startsWith("L") && classSignature.endsWith(";");
             return classSignature.substring(1, classSignature.length()-1)
                 .replace('/', '.');
         }