When encoding a 24 bit multichannel W64 file on a 32 bit system, a
calculation could overflow resulting in a "got partial sample"
error and early termination of the encoding process.

The fix was to replace the cast to `size_t` (which is only 32 bits
on a 32 bit system) with a cast to `uin64_t`.

Thanks to Janne Hyvärinen for helping to track down the problem.

Closes: https://sourceforge.net/p/flac/bugs/453