Add frame for loading and storing buffered messages

This commit introduces the frame for saving and loading
buffered messages within message reading function
mbedtls_ssl_read_record().
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 85ab722..3a8dd21 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -121,6 +121,7 @@
 #define MBEDTLS_ERR_SSL_INVALID_VERIFY_HASH               -0x6600  /**< Couldn't set the hash for verifying CertificateVerify */
 #define MBEDTLS_ERR_SSL_CONTINUE_PROCESSING               -0x6580  /**< Internal-only message signaling that further message-processing should be done */
 #define MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS                 -0x6500  /**< The asynchronous operation is not completed yet. */
+#define MBEDTLS_ERR_SSL_EARLY_MESSAGE                     -0x6480  /**< Internal-only message signaling that a message arrived early. */
 
 /*
  * Various constants