[findBugs] Fix potential NPE in ObjectDirectoryTest

Change-Id: I0307d1e9bbc0bfc8172eed10f8e93788d6e2c8b4
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/ObjectDirectoryTest.java

@@ -162,7 +162,7 @@ public class ObjectDirectoryTest extends RepositoryTestCase {
 					return name.endsWith(".pack");
 				}
 			});
-			assertTrue(ret.length == 1);
+			assertTrue(ret != null && ret.length == 1);
 			Assume.assumeTrue(tmpFile.lastModified() == ret[0].lastModified());
 
 			// all objects are in a new packfile but we will not detect it