[crypto] Check for all-zeros result from X25519 key exchange

RFC7748 states that it is entirely optional for X25519 Diffie-Hellman
implementations to check whether or not the result is the all-zero
value (indicating that an attacker sent a malicious public key with a
small order).  RFC8422 states that implementations in TLS must abort
the handshake if the all-zero value is obtained.

Return an error if the all-zero value is obtained, so that the TLS
code will not require knowledge specific to the X25519 curve.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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