Fix <= TLS1.2 break

Changing the value of SSL_MAX_MASTER_KEY_LENGTH had some unexpected
side effects in the <=TLS1.2 code which apparently relies on this being
48 for interoperability. Therefore create a new define for the TLSv1.3
resumption master secret which can be up to 64 bytes.

Found through the boring test suite.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2259)