[spotbugs] Fix potential NPE in GcPruneNonReferencedTest

File#listFiles can return null, assert it is not null to fix the
warning.

Change-Id: I28fc668fee760d39965e6e039003ac9f85fd461b
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/GcPruneNonReferencedTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/GcPruneNonReferencedTest.java
index 796df3d..7386621 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/GcPruneNonReferencedTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/GcPruneNonReferencedTest.java
@@ -12,6 +12,7 @@
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertTrue;
 
 import java.io.File;
@@ -89,6 +90,7 @@
 
 	private void assertNoEmptyFanoutDirectories() {
 		File[] fanout = repo.getObjectsDirectory().listFiles();
+		assertNotNull(fanout);
 		for (File f : fanout) {
 			if (f.isDirectory()) {
 				String[] entries = f.list();