Reset sudo password after failed Unix elevation

This allows to request the sudo password from user again on next
elevation attempt without any need for full application restart.

v2rayN/ServiceLib/Handler/CoreAdminHandler.cs

@@ -64,6 +64,7 @@ public class CoreAdminHandler
         catch (Exception ex)
         {
             _linuxSudoPid = -1;
+            AppHandler.Instance.LinuxSudoPwd = "";
             throw new Exception(ResUI.FailedToRunCore, ex);
         }