Fix potential null pointer access in IndexDiffFilter

Change-Id: I39c2ea6b3090d7028cc1e246af6e3cc4e7bc70c2
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/treewalk/filter/IndexDiffFilter.java

@@ -224,7 +224,8 @@ public class IndexDiffFilter extends TreeFilter {
 		// Only one chance left to detect a diff: between index and working
 		// tree. Make use of the WorkingTreeIterator#isModified() method to
 		// avoid computing SHA1 on filesystem content if not really needed.
-		return wi.isModified(di.getDirCacheEntry(), true, tw.getObjectReader());
+		return wi.isModified(di == null ? null : di.getDirCacheEntry(), true,
+				tw.getObjectReader());
 	}
 
 	/**