Fix timer pointer not being reset when a timer is killed
Fix timer pointer not being reset when a timer is killed

The timer pointer is compared against NULL in timer_process_timers()
but in fact it is never set to NULL once create, even when the memory
is freed by timer_kill()!!

Zeex/sampgdk,WopsS/sampgdk,WopsS/sampgdk,Zeex/sampgdk,WopsS/sampgdk,Zeex/sampgdk