More correct calculation size for copying full memory buffers

[GermanAizek]

@jsteube, carefully review code, I could have made inconspicuous mistake.

[ventaquil]

How about it?

https://github.com/hashcat/hashcat/pull/4039/files#diff-38a9c12141e4dd7627bb7684fca8f8e319c45a300f9babd390b6548c77a6133aL231