Bug#14234028 - CRASH DURING SHUTDOWN WITH BACKGROUND PURGE THREAD

Analysis
 --------- 
 
 my_stat() calls stat() and if the stat() call fails we try to set 
 the variable  my_errno which is actually a thread specific data .
 We try to get the  address of this thread specific data using
 my_pthread_getspecifc(),but for the purge thread we have not defined 
 any thread specific data so it returns null and when dereferencing 
 null we get a segmentation fault.
        init_available_charsets() seen in the core stack is invoked 
 through  pthread_once() .pthread_once is used for one time 
 initialization.Since free_charsets() is called before innodb plugin 
 shutdown ,purge thread calls init_avaliable_charsets() which leads 
 to the crash.

 Fix
 ---
 Call free_charsets() after the innodb plugin shutdown,since purge 
 threads are still using the charsets.

sql/mysqld.cc

@@ -1476,7 +1476,6 @@ void clean_up(bool print_message)
   item_user_lock_free();
   lex_free();				/* Free some memory */
   item_create_cleanup();
-  free_charsets();
   if (!opt_noacl)
   {
 #ifdef HAVE_DLOPEN
@@ -1532,6 +1531,7 @@ void clean_up(bool print_message)
   logger.cleanup_end();
   my_atomic_rwlock_destroy(&global_query_id_lock);
   my_atomic_rwlock_destroy(&thread_running_lock);
+  free_charsets();
   mysql_mutex_lock(&LOCK_thread_count);
   DBUG_PRINT("quit", ("got thread count lock"));
   ready_to_exit=1;