If your client is willing to pay for it, you escrow your code with a neutral third party. There are many firms that will provide this escrow service. You write a contact that specifies exactly when and how the source code will be released from escrow.
You basically place your software package's source code with a neutral third party. This party will release the source code to your customer under certain specified conditions. (One example might be your company goes bankrupt.)
There are hundreds of companies that provide this service. Here are just a few.
Escrow Tech International
Typically you sign a contract that specifies under which conditions your software source code will be released to the other party. These services charge a fee. You typically have the big company, that is worried about what happens if you vanish, pay for the escrow fee. You also need a attorney to review this contract and you need to consider those legal fees as part of your bid to this company.
Finally, note that while your client may be quite happy with the escrowed software, programmers know that the source code is only one minor portion of a product. Getting something to work with just the source code is going to be difficult at best.
access to source code if the start up fails