Introduce PSA_KEY_HANDLE_INIT macro

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/library/pk_wrap.c b/library/pk_wrap.c
index 903c53b..a40734b 100644
--- a/library/pk_wrap.c
+++ b/library/pk_wrap.c
@@ -543,7 +543,7 @@
     mbedtls_ecdsa_context *ctx = ctx_arg;
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
-    psa_key_handle_t key_handle = 0;
+    psa_key_handle_t key_handle = PSA_KEY_HANDLE_INIT;
     psa_status_t status;
     mbedtls_pk_context key;
     int key_len;