Make PutNextEntry throw if the entry is AES and no password has been set.

[Numpsy]

refs #507 - possible simple way to avoid the issue, by making PutNextEntry throw if the new entry has an AESKeySize set and the stream password hasn't been set.

I made it an InvalidOperationException rather than an ArgumentException with the idea that the operation is invalid when the stream has no password,

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[piksel]

Yeah, the argument isn't actually the problem, but rather the state of the ZipOutputStream (even though you could argue that the AESKeySize of the argument is the issue, but I think that's less helpful).