RT2943: Check sizes if -iv and -K arguments

RT2943 only complains about the incorrect check of -K argument size,
we might as well do the same thing with the -iv argument.

Before this, we only checked that the given argument wouldn't give a
bitstring larger than EVP_MAX_KEY_LENGTH.  we can be more precise and
check against the size of the actual cipher used.

Reviewed-by: Rich Salz <rsalz@openssl.org>