Fix memory corruption with odb backend

Author: jairbubbles

LibGit2Sharp/Core/GitOdbBackend.cs

@@ -33,6 +33,9 @@ static GitOdbBackend()
         public exists_prefix_callback ExistsPrefix;
         public IntPtr Refresh;
         public foreach_callback Foreach;
+
+        private IntPtr Padding; // TODO: add writemidx
+
         public IntPtr Writepack;
         public IntPtr Freshen;
         public free_callback Free;