Avoid an NPE after 7b01a53692

While fixing an NPE, I introduced another one in a deprecated isModified
method. It cannot avoid NPE's entirely, which is the reason the method
is deprecated

Change-Id: I5147c1c94621586dd84bd11e6090a954523b6c1c

org.eclipse.jgit/src/org/eclipse/jgit/treewalk/WorkingTreeIterator.java

@@ -803,7 +803,8 @@ public abstract class WorkingTreeIterator extends AbstractTreeIterator {
 	 */
 	public boolean isModified(DirCacheEntry entry, boolean forceContentCheck) {
 		try {
-			return isModified(entry, forceContentCheck, null);
+			return isModified(entry, forceContentCheck,
+					repository.newObjectReader());
 		} catch (IOException e) {
 			throw new JGitInternalException(e.getMessage(), e);
 		}