Avoid a NULL dereference in the DTLS client that can be triggered by a
crafted server response used in conjunction with an anonymous DH or
anonymous ECDH ciphersuite.

Fixes CVE-2014-3510, which is effectively a repeat of CVE-2014-3470 in
copied code.

Reported by Felix Groebert of the Google Security Team.

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Avoid a double-free in an error path.

Reported by Felix Groebert of the Google Security Team.

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Ensure that sess_cert is not NULL at the start of
ssl3_send_client_key_exchange(), rather than checking it in the key
exchange algorithm specific code.

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