BIO: at the end of BIO_new, declare the BIO inited if no create method present

Without this, every BIO implementation is forced to have a create
method, just to set bio->init = 1.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5223)

(cherry picked from commit 7f55808f)