Fix for infinite loop in smokeFlow() proc.

When chemical grenade being activated on any unsimulated turf (for example a space turf), smokeFlow() starts an infinite while loop, overloading the server.
With the only one unsimulated turf in pending list while() loop can't finish itself and for() loop can't start.
This fix makes chemical grenades being able to explode in space without server overload while not affecting their current game logic.