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.

Closes: #229

src/rf64.c

@@ -742,7 +742,7 @@ rf64_write_header (SF_PRIVATE *psf, int calc_length)
 
 	pad_size = psf->dataoffset - 16 - psf->header.indx ;
 	if (pad_size >= 0)
-		psf_binheader_writef (psf, "m4z", PAD_MARKER, pad_size, make_size_t (pad_size)) ;
+		psf_binheader_writef (psf, "m4z", PAD_MARKER, (unsigned int) pad_size, make_size_t (pad_size)) ;
 
 	if (wpriv->rf64_downgrade && (psf->filelength < RIFF_DOWNGRADE_BYTES))
 		psf_binheader_writef (psf, "tm8", data_MARKER, psf->datalength) ;