Introduce PSA_KEY_HANDLE_INIT macro

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index 31d6000..1dd2642 100644
--- a/tests/suites/test_suite_x509write.function
+++ b/tests/suites/test_suite_x509write.function
@@ -161,7 +161,7 @@
                                  int cert_type )
 {
     mbedtls_pk_context key;
-    psa_key_handle_t slot = 0;
+    psa_key_handle_t slot = PSA_KEY_HANDLE_INIT;
     psa_algorithm_t md_alg_psa;
     mbedtls_x509write_csr req;
     unsigned char buf[4096];