Introduce PSA_KEY_HANDLE_INIT macro

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 76be8ab..9494c65 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -3802,7 +3802,7 @@
         status = psa_destroy_key( handshake->ecdh_psa_privkey );
         if( status != PSA_SUCCESS )
             return( MBEDTLS_ERR_SSL_HW_ACCEL_FAILED );
-        handshake->ecdh_psa_privkey = 0;
+        handshake->ecdh_psa_privkey = PSA_KEY_HANDLE_INIT;
     }
     else
 #endif /* MBEDTLS_USE_PSA_CRYPTO &&