If it's your own source code, Nasm claims *no* rights for processing it. You can do anything you want with it. (We hope you *will* distribute source, but it isn't required.)
If you "borrow" some code from Nasm - you'd have to "translate" it from C to asm - *then* the LGPL kicks in. I don't think you have to worry about it.
I am not a lawyer and this is not legal advice.
Best,
Frank