Fix Issue #71 & Other undocumented error

- Added changing of firstLoopOfAction boolean to false in shoot action,
it was causing null reference exceptions when a unit died.
- Added checks to ensure sustained fire sprites are actually there
before trying to change or destroy them, this was causing null reference
exceptions.
- Assigned parent transforms of sustained fire sprites to the relevant
unit gameobject, so that sustained fire sprites will move with their
gameobject if it moves (e.g. if a genestealer moves)