FileCacheStore throws an IllegalMonitorStateException in FileCacheVamTest

If there are no expired elements the bucket will not be modified and the lock doesn't need to be acquired.
We were still releasing the lock though, leading to IllegalMonitorStateExceptions in FileCacheStoreVamTest:

2011-09-25 11:54:46,000 ERROR (testng-FileCacheStoreVamTest) [org.infinispan.test.fwk.UnitTestTestNGListener] Method testLoadAndStoreWithLifespan(org.infinispan.loaders.file.FileCacheStoreVamTest) threw an exception
java.lang.IllegalMonitorStateException
at java.util.concurrent.locks.ReentrantReadWriteLock$Sync.tryReleaseShared(ReentrantReadWriteLock.java:398)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.releaseShared(AbstractQueuedSynchronizer.java:1340)
at java.util.concurrent.locks.ReentrantReadWriteLock$ReadLock.unlock(ReentrantReadWriteLock.java:778)
at org.infinispan.util.concurrent.locks.StripedLock.releaseLock(StripedLock.java:135)
at org.infinispan.loaders.LockSupportCacheStore.unlock(LockSupportCacheStore.java:87)
at org.infinispan.loaders.file.FileCacheStore.purgeInternal(FileCacheStore.java:244)
at org.infinispan.loaders.AbstractCacheStore$2.run(AbstractCacheStore.java:106)
at org.infinispan.util.concurrent.WithinThreadExecutor.execute(WithinThreadExecutor.java:48)
at org.infinispan.loaders.AbstractCacheStore.purgeExpired(AbstractCacheStore.java:103)
at org.infinispan.loaders.BaseCacheStoreTest.purgeExpired(BaseCacheStoreTest.java:198)
at org.infinispan.loaders.BaseCacheStoreTest.testLoadAndStoreWithLifespan(BaseCacheStoreTest.java:151)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:616)
at org.testng.internal.MethodHelper.invokeMethod(MethodHelper.java:644)
at org.testng.internal.Invoker.invokeMethod(Invoker.java:546)
at org.testng.internal.Invoker.invokeTestMethod(Invoker.java:700)
at org.testng.internal.Invoker.invokeTestMethods(Invoker.java:1002)
at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:137)
at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:121)
at org.testng.TestRunner.runWorkers(TestRunner.java:909)
at org.testng.TestRunner.privateRun(TestRunner.java:618)
at org.testng.TestRunner.run(TestRunner.java:499)
at org.testng.SuiteRunner.runTest(SuiteRunner.java:332)
at org.testng.SuiteRunner.access$000(SuiteRunner.java:33)
at org.testng.SuiteRunner$SuiteWorker.run(SuiteRunner.java:358)
at org.testng.internal.thread.ThreadUtil$CountDownLatchedRunnable.run(ThreadUtil.java:142)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:679)