Check for negative IV length

Check for negative IV length

A recent change in EVP_CIPHER_CTX_iv_length() made it possible in principle
that this function returns -1. This can only happen for an incorrectly set
up EVP_CIPHER. Still it is better form to check for negative lengths before
stuffing it into a memcpy().

It would probably be desirable to cap the iv_length to something large
enough. This can be done another time.

ok beck