Fix bug in BIO_f_linebuffer()

In BIO_f_linebuffer, this would cause an error:

    BIO_write(bio, "1\n", 1);

I.e. there's a \n just after the part of the string that we currently
ask to get written.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5353)