BUG#22867809 - MY_WRITE, MY_PWRITE NO LONGER SAFE TO CALL

               FROM THD-LESS SERVER UTILITY THREADS.

If my_write system write calls writes fewer bytes than
requested, the code will check whether the current connection
 has received a KILL. If my_write is called from a server
utility thread, there is no THD context associated with it.
Hence current_thd doesn't exist and it will fail.

The fix is to check current_thd before using it.

Author: thayumanavar77

sql/sql_class.cc

@@ -3762,7 +3762,7 @@ void THD::end_attachable_transaction()
 extern "C" int thd_killed(const MYSQL_THD thd)
 {
   if (thd == NULL)
-    return current_thd->killed;
+    return current_thd != NULL ? current_thd->killed : 0;
   return thd->killed;
 }