[nrf fromtree] Clarify the need for calling mbedtls_ssl_derive_keys a…

…fter extension parsing

Use a more straightforward condition to note that session resumption
is happening.

Co-authored-by: Ronald Cron <ronald.cron@arm.com>
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
Signed-off-by: Georgios Vasilakis <georgios.vasilakis@nordicsemi.no>

(cherry picked from commit 21b5080)

library/ssl_cli.c

@@ -2368,7 +2368,12 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
         }
     }
 
-    if( ssl->state == MBEDTLS_SSL_SERVER_CHANGE_CIPHER_SPEC )
+    /*
+     * mbedtls_ssl_derive_keys() has to be called after the parsing of the
+     * extensions. It sets the transform data for the resumed session which in
+     * case of DTLS includes the server CID extracted from the CID extension.
+     */
+    if( ssl->handshake->resume )
     {
         if( ( ret = mbedtls_ssl_derive_keys( ssl ) ) != 0 )
         {