[spotbugs] Fix potential NPE in PackFileSnapshotTest

Path#getFileName can return null. Fix the warning by asserting the file
name isn't null.

Change-Id: I7f2fe75b46113d8be1d14e3f18dd77da27df25ed
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

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

 
 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 static org.junit.Assume.assumeFalse;
 import static org.junit.Assume.assumeTrue;
 		// Repack to create initial packfile. Make a copy of it
 		PackFile pf = repackAndCheck(5, null, null, null);
 		Path packFilePath = pf.getPackFile().toPath();
-		Path packFileBasePath = packFilePath.resolveSibling(
-				packFilePath.getFileName().toString().replaceAll(".pack", ""));
+		Path fn = packFilePath.getFileName();
+		assertNotNull(fn);
+		String packFileName = fn.toString();
+		Path packFileBasePath = packFilePath
+				.resolveSibling(packFileName.replaceAll(".pack", ""));
 		AnyObjectId chk1 = pf.getPackChecksum();
 		String name = pf.getPackName();
 		Long length = Long.valueOf(pf.getPackFile().length());