fix main.lock_kill crashes in --ps --embed

when checking whether thd wasn't killed before this
emb_advanced_command(), take into account that it
could've been killed before the *previous*
emb_advanced_command(). That is, the previous one has
already set thd to NULL and this one only wanted a COM_STMT_RESET
after a failure.

libmysqld/lib_sql.cc

@@ -115,7 +115,7 @@ emb_advanced_command(MYSQL *mysql, enum enum_server_command command,
   NET *net= &mysql->net;
   my_bool stmt_skip= stmt ? stmt->state != MYSQL_STMT_INIT_DONE : FALSE;
 
-  if (thd->killed != NOT_KILLED)
+  if (thd && thd->killed != NOT_KILLED)
   {
     if (thd->killed < KILL_CONNECTION)
       thd->killed= NOT_KILLED;