Deal with failure of crypt_keyslot_max

src/api/encryption_keys_get.c

@@ -34,7 +34,14 @@ int callback_encryption_keys_get(const struct _u_request * request,
     bool in_use;
     bool last_used;
     int keyslot = 0;
-    for (keyslot = 0; keyslot <= crypt_keyslot_max(CRYPT_LUKS1); keyslot++)
+    int keyslot_max = crypt_keyslot_max(CRYPT_LUKS1);
+    if (keyslot_max < 0)
+    {
+        return send_simple_response(response, 500, "error",
+            "crypt_keyslot_max failed");
+    }
+
+    for (keyslot = 0; keyslot < keyslot_max; keyslot++)
     {
         // Create new json object containing info for this keyslot.
         key = json_object();