pk: fix library code for using the new opaque key solution

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/tests/src/test_helpers/ssl_helpers.c b/tests/src/test_helpers/ssl_helpers.c
index e79d152..23f5977 100644
--- a/tests/src/test_helpers/ssl_helpers.c
+++ b/tests/src/test_helpers/ssl_helpers.c
@@ -595,8 +595,7 @@
         if (cert->pkey != NULL) {
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
             if (mbedtls_pk_get_type(cert->pkey) == MBEDTLS_PK_OPAQUE) {
-                mbedtls_svc_key_id_t *key_slot = cert->pkey->pk_ctx;
-                psa_destroy_key(*key_slot);
+                psa_destroy_key(cert->pkey->opaque_id);
             }
 #endif
             mbedtls_pk_free(cert->pkey);