BIO: prevent crash on NULL BIO for prefix_ctrl() and thus for BIO_set_prefix(), BIO_set_indent(), etc.
BIO: prevent crash on NULL BIO for prefix_ctrl() and thus for BIO_set_prefix(), BIO_set_indent(), etc.

Reviewed-by: Tomas Mraz <2bc6038c3dfca09b2da23c8b6da8ba884dc2dcc2@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15722)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl