Fix vehicles segfaulting from parts breaking off

Fixes #4497 (definitely), fixes #4549 (I think, it sounds like he had the same problem but I haven't actively confirmed), and obsoletes #4499.

* Vehicles destroyed by damage (as opposed to disassembly) will be properly removed from the map.
* Parts breaking off will refresh the vehicle cache, so the vehicle will no longer look like someone took a car-sized eggbeater to it.

With thanks to @Skyyrunner who dug up enough clues for me to figure out what was going wrong, and also because obliterating vehicles with incendiary ammunition is ridiculous amounts of fun.