Chemsmoke tweaks and bug fixes

- Renamed the variable for the smoke density mechanic to density. Also clamped it to a minimum of 1 so it cant multiply reagents.
- Added some checks so the chemsmoke effect doesn't run if it's in nullspace.
- Removed an unnecessary distance calculation.
- Added a null turf check to prevent runtimes if a really big smoke grenade goes off at the edge of the map.
- Added a check so the reagent copy code doesn't run if there's no reagents to run it on.