Check parentFile is not null

parentFile becomes null when f is relative path, such as ".".

This patch avoids NullPointerException in such case.

Change-Id: I4752674b1daab6eedd7c3650c7749462810eaffd
Signed-off-by: Hiroshi Tomita <tomykaira@gmail.com>

org.eclipse.jgit/src/org/eclipse/jgit/dircache/DirCacheCheckout.java

@@ -475,7 +475,7 @@ public class DirCacheCheckout {
 	 private void removeEmptyParents(File f) {
 		File parentFile = f.getParentFile();
 
-		while (!parentFile.equals(repo.getWorkTree())) {
+		while (parentFile != null && !parentFile.equals(repo.getWorkTree())) {
 			if (!parentFile.delete())
 				break;
 			parentFile = parentFile.getParentFile();