stop background writer thread when panic() happened on it

h2/src/main/org/h2/mvstore/MVStore.java

@@ -2845,10 +2845,10 @@ public class MVStore implements AutoCloseable {
         // which should not happen with non-weak flavour of CAS operation,
         // but I've seen it, so just to be safe...
         BackgroundWriterThread t;
-        while ((t = backgroundWriterThread.get()) != null &&
-                // if called from within the thread itself - can not join
-                t != Thread.currentThread()) {
+        while ((t = backgroundWriterThread.get()) != null) {
             if (backgroundWriterThread.compareAndSet(t, null)) {
+                // if called from within the thread itself - can not join
+                if (t != Thread.currentThread()) {
                     synchronized (t.sync) {
                         t.sync.notifyAll();
                     }
@@ -2860,6 +2860,7 @@ public class MVStore implements AutoCloseable {
                             // ignore
                         }
                     }
+                }
                 break;
             }
         }