Introduce psa_key_handle_is_null inline function

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 85d9df4..5d9b34e 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1300,7 +1300,7 @@
     psa_se_drv_table_entry_t *driver;
 #endif /* MBEDTLS_PSA_CRYPTO_SE_C */
 
-    if( handle == 0 )
+    if( psa_key_handle_is_null( handle ) )
         return( PSA_SUCCESS );
 
     status = psa_get_key_slot( handle, &slot );