Use more appropriate DatumGet* function

Use more appropriate DatumGet* function

Use DatumGetCString() instead of DatumGetPointer() for returning a C
string.  Right now, they are the same, but that doesn't always have to
be so.

Reviewed-by: Chao Li <[email protected]>
Reviewed-by: Bertrand Drouvot <[email protected]>
Discussion: https://www.postgresql.org/message-id/4154950a-47ae-4223-bd01-1235cc50e933%40eisentraut.org