CopyFile: fix BufSize to a sane value

CopyFile: fix BufSize to a sane value

Commit e977b8b9234ac5db32f2f0ad7e183139b988340d tries to make BufSize
calculated based on the size of the buffer; the problem is that
std::unique_ptr::size() returns a pointer to the data, so sizeof()
equals to the size of a pointer (later divided by sizeof(char), which
is 1). The result is that the CopyFile copies in chunks of 8 bytes,
which is not exactly ideal...

As solution, declare BufSize in advance, and use its value to allocate
the Buf array.

Closes: #808381