Bug#19553099 EXPIRE_LOGS_DAYS=1 CRASH MYSQLD DURING RESTART, DBUG_SYNC(NULL, ...)

 - Restarting mysqld with --expire-log-days=1 triggers 'log_in_use()' to be called while current_thd is NULL.
 - Check current_thd before calling DEBUG_SYNC() to avoid passing NULL pointer to DEBUG_SYNC()
 - Wrap debug code construct inside #ifndef DBUG_OFF like in other parts of the file

sql/sql_repl.cc

@@ -248,7 +248,10 @@ bool log_in_use(const char* log_name)
   size_t log_name_len = strlen(log_name) + 1;
   THD *tmp;
   bool result = 0;
-  DEBUG_SYNC(current_thd,"purge_logs_after_lock_index_before_thread_count");
+#ifndef BDUG_OFF
+  if (current_thd)
+    DEBUG_SYNC(current_thd,"purge_logs_after_lock_index_before_thread_count");
+#endif
   mysql_mutex_lock(&LOCK_thread_count);
   I_List_iterator<THD> it(threads);