MDEV-6619 SHOW PROCESSLIST returns empty result set after KILL QUERY

don't send an OK packet if the SHOW PROCESSLIST was killed

mysql-test/r/kill_processlist-6619.result

+connect  con1,localhost,root,,;
+SHOW PROCESSLIST;
+Id	User	Host	db	Command	Time	State	Info	Progress
+#	root	#	test	Sleep	#	#	NULL	0.000
+#	root	#	test	Query	#	#	SHOW PROCESSLIST	0.000
+connection default;
+KILL QUERY con_id;
+connection con1;
+SHOW PROCESSLIST;
+ERROR 70100: Query execution was interrupted
+SHOW PROCESSLIST;
+Id	User	Host	db	Command	Time	State	Info	Progress
+#	root	#	test	Sleep	#	#	NULL	0.000
+#	root	#	test	Query	#	#	SHOW PROCESSLIST	0.000

mysql-test/t/kill_processlist-6619.test

+#
+# MDEV-6619 SHOW PROCESSLIST returns empty result set after KILL QUERY
+#
+--source include/not_embedded.inc
+--enable_connect_log
+--connect (con1,localhost,root,,)
+--let $con_id = `SELECT CONNECTION_ID()`
+--replace_column 1 # 3 # 6 # 7 #
+SHOW PROCESSLIST;
+--connection default
+--replace_result $con_id con_id
+eval KILL QUERY $con_id;
+--connection con1
+--error ER_QUERY_INTERRUPTED
+SHOW PROCESSLIST;
+--replace_column 1 # 3 # 6 # 7 #
+SHOW PROCESSLIST;

sql/sql_show.cc

@@ -2178,9 +2178,10 @@ void mysqld_list_processes(THD *thd,const char *user, bool verbose)
                             Protocol::SEND_NUM_ROWS | Protocol::SEND_EOF))
     DBUG_VOID_RETURN;
 
+  if (thd->killed)
+    DBUG_VOID_RETURN;
+
   mysql_mutex_lock(&LOCK_thread_count); // For unlink from list
-  if (!thd->killed)
-  {
   I_List_iterator<THD> it(threads);
   THD *tmp;
   while ((tmp=it++))
@@ -2250,7 +2251,6 @@ void mysqld_list_processes(THD *thd,const char *user, bool verbose)
       thread_infos.append(thd_info);
     }
   }
-  }
   mysql_mutex_unlock(&LOCK_thread_count);
 
   thread_info *thd_info;