It's undefined behavior to pass a null pointer to std::fread and
std::fwrite, even if the length passed in is zero, so we must perform
the precondition checking ourselves.
A common case where this can occur is when passing in the data of an
empty std::vector and size, as an empty vector will typically have a
null internal buffer.
While we're at it, we can move the implementation out of line and add
debug checks against passing in nullptr to std::fread and std::fwrite.
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