Don't allow 8bytes for data file pointers for now.

myisam/mi_create.c

@@ -723,10 +723,13 @@ int mi_create(const char *name,uint keys,MI_KEYDEF *keydefs,
 
 uint mi_get_pointer_length(ulonglong file_length, uint def)
 {
+  DBUG_ASSERT(def >= 2 && def <= 7);
   if (file_length)				/* If not default */
   {
+#ifdef NOT_YET_READY_FOR_8_BYTE_POINTERS
     if (file_length >= (longlong) 1 << 56)
       def=8;
+#endif
     if (file_length >= (longlong) 1 << 48)
       def=7;
     if (file_length >= (longlong) 1 << 40)