Analysis:
When we access fts_internal_tbl_name in i_s_fts_config_fill (),
it can be set to NULL by another session.

Solution:
Define fts_internal_tbl_name2 for global variable innodb_ft_aux_table,
if it's NULL, set fts_internal_tbl_name to "default".
Reviewed-by: Jimmy Yang <jimmy.yang@oracle.com>
RB: 13401