Fix NPE in DfsPackCompactor.compactReftables

Output pack description needs to be initialized.

Change-Id: I4b66a18fa7020f528b312bc9f74b8d094ad6ad66
Signed-off-by: Minh Thai <mthai@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsPackCompactor.java

@@ -311,9 +311,9 @@ public class DfsPackCompactor {
 		DfsObjDatabase objdb = repo.getObjectDatabase();
 		Collections.sort(srcReftables, objdb.reftableComparator());
 
+		initOutDesc(objdb);
 		try (DfsReftableStack stack = DfsReftableStack.open(ctx, srcReftables);
 		     DfsOutputStream out = objdb.writeFile(outDesc, REFTABLE)) {
-			initOutDesc(objdb);
 			ReftableCompactor compact = new ReftableCompactor(out);
 			compact.addAll(stack.readers());
 			compact.setIncludeDeletes(true);