add helper method IsIndexValid

[Alexey-T]

adding helper func will help in places like this:

function THasherList.GetName(Index: Integer): String;
begin
  if (Index > FList.Count-1) or (Index < 0) then Result := ''

[PascalVault]

This is a very simple IF. Replacing it with a method will make the slower.

[Alexey-T]

then add inline.