modes/ocb128.c: Reset nonce-dependent variables on setiv

Upon a call to CRYPTO_ocb128_setiv, either directly on an OCB_CTX or
indirectly with EVP_CTRL_AEAD_SET_IVLEN, reset the nonce-dependent
variables in the OCB_CTX.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/6420)