Set psk to NULL in ssl_psk_remove-backport to 2.28

Summary:
Back port [PR 9241](#9241) to
2.28 branch

Test Plan:

Reviewers:

Subscribers:

Tasks:

Tags:

library/ssl_tls.c

@@ -4452,6 +4452,7 @@ static void ssl_remove_psk(mbedtls_ssl_context *ssl)
                                  ssl->handshake->psk_len);
         mbedtls_free(ssl->handshake->psk);
         ssl->handshake->psk_len = 0;
+        ssl->handshake->psk = NULL;
     }
 }