Fix possible NPE in DirCache.isModified()

The snapshot field of a DirCache object for a newly created repository
can be null. This fix prevents a NPE when isModified() is called in
such a situation.

Change-Id: I61a1b45db2d60fdcc0f87373ac6fd75ac4c4a202
Signed-off-by: Philipp Thun <philipp.thun@sap.com>

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

@@ -321,7 +321,7 @@ public class DirCache {
 	public boolean isOutdated() throws IOException {
 		if (liveFile == null || !liveFile.exists())
 			return false;
-		return snapshot.isModified(liveFile);
+		return snapshot != null && snapshot.isModified(liveFile);
 	}
 
 	/** Empty this index, removing all entries. */