npe in memory leak verifier

Author: wumpz

src/test/java/net/sf/jsqlparser/test/MemoryLeakVerifier.java

@@ -101,7 +101,7 @@ private static void assertGarbageCollected(WeakReference<Object> ref, int maxIte
         for (int i = 0; i < maxIterations; i++) {
             runtime.runFinalization();
             runtime.gc();
-            if (ref.get() == null) {
+            if (ref == null || ref.get() == null) {
                 break;
             }