OpenSSL: Clean up TLS PRF implementation

OpenSSL: Clean up TLS PRF implementation

Commit fa0e715100b0e6fd956b6de67c3cdf908437436a ('Use
tls_connection_prf() for all EAP TLS-based key derivation') copied some
pointer checks from the generic implementation to tls_openssl.c.
However, these are arrays and cannot be NULL in OpenSSL data. Remove the
unnecessary checks and add master_key_length check for completeness.
(CID 109619).

Signed-off-by: Jouni Malinen <j@w1.fi>