Check CallbackReference#cbstruct for null when checking existing Refe…

…rence

The CallbackReference is held in a WeahHashMap, where it might be
removed from when it becomes eligable for GC. For this there is a null
guard in place, but when the callback reference is explicitly closed,
the associated native structure is freed and "nulled", this case also
needs to be guarded.

src/com/sun/jna/CallbackReference.java

@@ -508,7 +508,7 @@ private static Pointer getFunctionPointer(Callback cb, boolean direct) {
         Map<Callback, CallbackReference> map = direct ? directCallbackMap : callbackMap;
         synchronized(pointerCallbackMap) {
             CallbackReference cbref = map.get(cb);
-            if (cbref == null) {
+            if (cbref == null || cbref.cbstruct == null) {
                 cbref = new CallbackReference(cb, callingConvention, direct);
                 map.put(cb, cbref);
                 pointerCallbackMap.put(cbref.getTrampoline(),