Extending time limit breaks game modes with mp_timelimit 0

[TheOGSlayered]

Pretty simple, in a comp setup the mp_timelimit is set to 0, if the timelimit step is > 0 then it will end the comp match immediately after the extend option wins an end of map vote. I run a multi mode csgo server so if the timestep is set to 0 then we cant extend our deathmatch timelimit (the only gamemode that needs a timelimit).

Please fix the code to check that mp_timelimit > 0 before adding to the time limit.