DHT: Fix NPE during prefetch

The Prefetcher may have loaded a chunk that is a fragment, if the
DhtReader is scanning the Prefetcher's chunks for a particular
object fragment chunks will be missing the index and NPE during
the findOffset() call into the index itself.

Change-Id: Ie2823724c289f745655076c5209acec32361a1ea
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit.storage.dht/src/org/eclipse/jgit/storage/dht/PackChunk.java

@@ -291,7 +291,7 @@ public final class PackChunk {
 	}
 
 	int findOffset(RepositoryKey repo, AnyObjectId objId) {
-		if (key.getRepositoryId() == repo.asInt())
+		if (key.getRepositoryId() == repo.asInt() && index != null)
 			return index.findOffset(objId);
 		return -1;
 	}