[C++] Cast memcpy argument to size_t

[trozen]

memcpy is expecting a size_t argument, 32-bit builds fail with the following message:

error: conversion to ‘size_t {aka unsigned int}’ from ‘uint64_t {aka long long unsigned int}’ may alter its value [-Werror=conversion]

Adding explicit cast to silence these errors/warnings.

(IMO it would be better to use size_t where byte arrays and offsets are referenced. However this would change the public interface, so didn't want to to this as part of this PR.)