Make the data field for get_record() const

Make the data field for get_record() const

Improves consistency with the QUIC rstream implementation - and improves
the abstraction between the TLS implementation and the abstract record
layer. We should not expect that the TLS implementation should be able to
change the underlying buffer. Future record layers may not expect that.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20404)