Fix incorrect default keysize for CAST ofb and cfb modes.

Fix incorrect default keysize for CAST ofb and cfb modes.

Fixes #11459

It was incorrectly using 8 bytes instead of 16 as the default.
This was verified by expanding the macros used in e_cast.c.
The issue occurs if EVP_CIPHER_CTX_set_key_length() is not called.
evp_test.c hides this issue as it always calls EVP_CIPHER_CTX_set_key_length() before
using EVP_CipherInit_ex(...., key, ..).

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11707)