Fix DER reading from stdin for BIO_f_readbuffer

Fix DER reading from stdin for BIO_f_readbuffer

Fixes #14559

The intitial implementation of the gets() function tried using the next bio's gets() function.
For a file BIO this returned incorrect data for binary data containing 0x00.
Just buffering all data during gets() did not work however since some
applications open and close the bio multiple times when dealing with pem
files containing multiple entries.. This does not work
when reading from stdin unless the data if buffered one byte at a time.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14599)