Use an empty BIO memory buffer with BoringSSL.
Use an empty BIO memory buffer with BoringSSL.

de5225d1 mistakenly switched a BIO_s_null to an empty mem-BIO in order
to allow BoringSSL to work. That worked for BoringSSL, but OpenSSL
treats an empty mem-BIO as an error and so that was switched back in
2fe55c8f.

This change uses an empty mem-BIO with BoringSSL again for the same
reasons, but guards the change with the preprocessor so that it doesn't
break OpenSSL.

Change-Id: If90b7a151bf124722d91f150b441e0c9f5b96b03

google/conscrypt,google/conscrypt,google/conscrypt,google/conscrypt,google/conscrypt,google/conscrypt