MDEV-5131 create_embedded_thd is not thread safe, libmysqld.

        The emb_free_embedded_thd() has the thread-unsafe code
        so should be 'mutexed' also.

libmysqld/lib_sql.cc

@@ -416,11 +416,13 @@ int emb_unbuffered_fetch(MYSQL *mysql, char **row)
 static void emb_free_embedded_thd(MYSQL *mysql)
 {
   THD *thd= (THD*)mysql->thd;
+  mysql_mutex_lock(&LOCK_thread_count);
   thd->clear_data_list();
   thread_count--;
   thd->store_globals();
   thd->unlink();
   delete thd;
+  mysql_mutex_unlock(&LOCK_thread_count);
   my_pthread_setspecific_ptr(THR_THD,  0);
   mysql->thd=0;
 }