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 <li.evan.chao@gmail.com>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://www.postgresql.org/message-id/4154950a-47ae-4223-bd01-1235cc50e933%40eisentraut.org

src/test/modules/test_resowner/test_resowner_basic.c

@@ -35,13 +35,13 @@ static const ResourceOwnerDesc string_desc = {
 static void
 ReleaseString(Datum res)
 {
-	elog(NOTICE, "releasing string: %s", DatumGetPointer(res));
+	elog(NOTICE, "releasing string: %s", DatumGetCString(res));
 }
 
 static char *
 PrintString(Datum res)
 {
-	return psprintf("test string \"%s\"", DatumGetPointer(res));
+	return psprintf("test string \"%s\"", DatumGetCString(res));
 }
 
 /* demonstrates phases and priorities between a parent and child context */