Improve length checks for oiv and iv

Improve length checks for oiv and iv

There are two unsigned char arrays of size EVP_MAX_IV_LENGTH to store the
IVs of block ciphers. In most modes, only iv is used, but in some modes iv
is modified and oiv is used to store the original IV. At the moment nothing
enforces that they are of the same length. Therefore make sure the correct
one or both are checked before writing to or reading from them.

ok miod