Fix a buffer overflow in TLS 1.2 ClientKeyExchange parsing. When
MBEDTLS_USE_PSA_CRYPTO is enabled, the length of the public key in an ECDH
or ECDHE key exchange was not validated. This could result in an overflow of
handshake->xxdh_psa_peerkey, overwriting further data in the handshake
structure or further on the heap.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>