Crypto: Fix -Wint-conversion warning

Fix -Wint-conversion warning when assign NULL to 'key'.

Signed-off-by: Summer Qin <summer.qin@arm.com>
Change-Id: I41fe5423ffdbdeb9517696ed04d5bffdc7327dd8
diff --git a/secure_fw/partitions/crypto/crypto_key.c b/secure_fw/partitions/crypto/crypto_key.c
index dd03417..6f0aea2 100644
--- a/secure_fw/partitions/crypto/crypto_key.c
+++ b/secure_fw/partitions/crypto/crypto_key.c
@@ -44,7 +44,7 @@
     /* Skip checking idx */
 
     handle_owner[idx].owner = TFM_INVALID_CLIENT_ID;
-    handle_owner[idx].key = NULL;
+    handle_owner[idx].key = (psa_key_id_t)0;
     handle_owner[idx].in_use = TFM_CRYPTO_NOT_IN_USE;
 }