RefUpdateTest: Add null check to prevent potential NPE

Change-Id: I64b1850c3bb80b09409044461ca3fdde1c46d4bb
Signed-off-by: David Pursehouse <david.pursehouse@sonymobile.com>

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

@@ -751,7 +751,7 @@ public class RefUpdateTest extends SampleDataRepositoryTestCase {
 			assertNull(db.resolve(toName));
 			assertEquals(oldFromLog.toString(), db.getReflogReader(fromName)
 					.getReverseEntries().toString());
-			if (oldHeadId != null)
+			if (oldHeadId != null && oldHeadLog != null)
 				assertEquals(oldHeadLog.toString(), db.getReflogReader(
 						Constants.HEAD).getReverseEntries().toString());
 		} finally {