decrypt use ECB mode, when the input less than 16 chars, decrypt error

[nb7123]

'BlockCopy' just copy 'KEYLEN' chars

[kokke]

This is by design. You need to pass the AES library valid inputs.
That means making sure you're using correctly sized buffers and correctly zero-pad them as well, if needed.

As the readme says: "There is no built-in error checking or protection from out-of-bounds memory access errors as a result of malicious input.".
The algorithm expects blocks with lengths divisible by 16 bytes.
If you want to encrypt less data than a full block, pad it or use a stream oriented cipher mode like OFB :)