Use try/finally to ensure oldValue is cleared

If any JS exception is thrown during event delivery the oldValue
should still be set to null to enable garbage collection.

Change-Id: I740db06e93162f86982ffd418c2072bfb4d5d75c

gerrit-gwtui/src/main/java/com/google/gerrit/client/ui/ListenableOldValue.java

@@ -23,8 +23,11 @@ public class ListenableOldValue<T> extends ListenableValue<T> {
   }
 
   public void set(final T value) {
-    oldValue = get();
-    super.set(value);
-    oldValue = null; // allow it to be gced before the next event
+    try {
+      oldValue = get();
+      super.set(value);
+    } finally {
+      oldValue = null; // allow it to be gced before the next event
+    }
   }
 }