Add mbedtls_ssl_get_session_pointer()

Avoid useless copy with mbedtls_ssl_get_session() before serialising.

Used in ssl_client2 for testing and demonstrating usage, but unfortunately
that means mbedtls_ssl_get_session() is no longer tested, which will be fixed
in the next commit.
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 30fd6ec..0722045 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -2387,7 +2387,7 @@
  *                 of session cache or session tickets.
  *
  * \see            mbedtls_ssl_session_load()
- * \see            mbedtls_ssl_get_session()
+ * \see            mbedtls_ssl_get_session_pointer()
  *
  * \param session  The session structure to be saved.
  * \param buf      The buffer to write the serialized data to. It must be a
@@ -2405,6 +2405,23 @@
                               size_t *olen );
 
 /**
+ * \brief          Get a pointer to the current session structure, for example
+ *                 to serialise it.
+ *
+ * \warning        Ownership of the session remains with the SSL context - the
+ *                 returned pointer must not be kept after the connection has
+ *                 ended or been renegotiated.
+ *
+ * \see            mbedtls_ssl_session_save()
+ *
+ * \param ssl      SSL context
+ *
+ * \return         A pointer to the current session if successful,
+ *                 NULL if no session is active.
+ */
+const mbedtls_ssl_session *mbedtls_ssl_get_session_pointer( const mbedtls_ssl_context *ssl );
+
+/**
  * \brief               Set the list of allowed ciphersuites and the preference
  *                      order. First in the list has the highest preference.
  *                      (Overrides all version-specific lists)