Fix for incorrect key size with 0 length keySize var

[UserJHansen]

I have found several ppt samples which don't have a proper keySize set.

Since 0 is an incorrect key size this change increases the chance that the file will still be readable.

[nolze]

Thank you so much for identifying this tricky issue!
I found that your observation aligns with the RC4 CryptoAPI Encryption specification: RC4 CryptoAPI Encryption.
I've made some updates to your commit based on this specification. I would appreciate it if you could verify that the updated code works with your ppt samples before we proceed with the merge.

[UserJHansen]

Yes, this correctly decrypts them

[nolze]

Great! Thank you again for your contribution. d6832ff