cpplib: improved macro argument collection

[Neil Booth]

Hi Matt,

Matt Kraai wrote:-

> This function will never return the first buffer in the list.
> I believe the preceding changes will fix this.

Well spotted.  I noticed this independently whilst doing some tuning
of memory usage; it mucks up the chain and we don't free anything.

I'll fix it, along with other enhancements, in the next patch.

Neil.