As oakmont said, it is quite unusual for a mould to leak, but if it is not very thick and some parts of the surface coating are missing then a leak can occur. Is it not possible to fill and repair those cracks? 

If adding some more resin and reinforcement to the reverse side doesn't fix things, you could opt to envelope bag the entire mould but this isn't ideal if you're looking to make several parts, due to the cured residue that will be left behind.

You can look to cover the reverse side with some breather cloth, covered with an additional section of bagging film and gaffa tape it to the edges. This should offer some protection that can be peeled away afterwards. If it's well sealed, you should't get too much resin curing on the back that you can't easily deal with

If your part is a 'one-off' adding the covering to the moulds' reverse side might not matter too much but adding some breather cloth and film should help prevent any unwanted bag punctures. I'd look to rough sand the reverse to remove any obvious sharp bits beforehand, at least, though