In commit 1bd681c8 we introduced
a work-around for the missing MDL protection when the internal
tables of FULLTEXT INDEX are dropped during DDL operations.
That work-around suffered from a race condition. A purge thread
could have narrowly passed purge_sys.check_stop_FTS() and then
(while holding dict_sys.mutex) acquire a table reference right
after fts_lock_table() determined that no references were being
held.

fts_lock_table(): Protect the reference check with dict_sys.mutex.

Thanks to Thirunarayanan Balathandayuthapani for repeating the
failure and testing the fix.