Don't directly access iv_size

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index f0067f4..36c96f2 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -8420,7 +8420,7 @@
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
         transform->ivlen = PSA_CIPHER_IV_LENGTH(key_type, alg);
 #else
-        transform->ivlen = cipher_info->iv_size;
+        transform->ivlen = mbedtls_cipher_info_get_iv_size(cipher_info);
 #endif /* MBEDTLS_USE_PSA_CRYPTO */
 
         /* Minimum length */