Don't directly access iv_size

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/pkcs12.c b/library/pkcs12.c
index a0966f6..8540de2 100644
--- a/library/pkcs12.c
+++ b/library/pkcs12.c
@@ -155,7 +155,7 @@
 
     if ((ret = pkcs12_pbe_derive_key_iv(pbe_params, md_type, pwd, pwdlen,
                                         key, keylen,
-                                        iv, cipher_info->iv_size)) != 0) {
+                                        iv, mbedtls_cipher_info_get_iv_size(cipher_info))) != 0) {
         return ret;
     }
 
@@ -171,7 +171,7 @@
         goto exit;
     }
 
-    if ((ret = mbedtls_cipher_set_iv(&cipher_ctx, iv, cipher_info->iv_size)) != 0) {
+    if ((ret = mbedtls_cipher_set_iv(&cipher_ctx, iv, mbedtls_cipher_info_get_iv_size(cipher_info))) != 0) {
         goto exit;
     }