get_value_index
Description:
Gets the value that is at the given index for the given tag in the given list.
Parameters:
| this | 
           a TagList  | 
      
| tag | 
           tag to read out  | 
      
| index | 
           number of entry to read out  | 
      
Returns:
| 
           The GValue for the specified entry or null if the tag wasn't available or the tag doesn't have as many entries  |