Parse and verify peer CRT chain in local variable

`mbedtls_ssl_parse_certificate()` parses the peer's certificate chain
directly into the `peer_cert` field of the `mbedtls_ssl_session`
structure being established. To allow to optionally remove this field
from the session structure, this commit changes this to parse the peer's
chain into a local variable instead first, which can then either be freed
after CRT verification - in case the chain should not be stored - or
mapped to the `peer_cert` if it should be kept. For now, only the latter
is implemented.
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index e76f4f8..7cd0d1c 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -331,6 +331,9 @@
         ssl_ecrs_cke_ecdh_calc_secret,  /*!< ClientKeyExchange: ECDH step 2 */
         ssl_ecrs_crt_vrfy_sign,         /*!< CertificateVerify: pk_sign()   */
     } ecrs_state;                       /*!< current (or last) operation    */
+#if defined(MBEDTLS_X509_CRT_PARSE_C)
+    mbedtls_x509_crt *ecrs_peer_cert;        /*!< The peer's CRT chain.     */
+#endif /* MBEDTLS_X509_CRT_PARSE_C */
     size_t ecrs_n;                      /*!< place for saving a length      */
 #endif
 #if defined(MBEDTLS_SSL_PROTO_DTLS)