src/rf64.c: Fix varargs related bug

src/rf64.c: Fix varargs related bug

C's <stargs.h> functionality isn't type checked so that passing an
`sf_count_t` (64 bits) by mistake in place of a `unit32_t` can cause
errors. This would be fine if it was an error on every architecture
and platform, but its not. This particular problem only manifested
on armhf and some other Arm architectures. It was not an issue on
32 bit x86.

I have now fixed variants of this same bug several times.

Change-Id: Ie0024ce14affeb6034071aa80c339da915f8b7e1
Closes: https://github.com/erikd/libsndfile/issues/229