Fix for 2 crashes - EnemyHealth.lua

[Vlad-00003]

This should fix 2 crasehs:

1. The mod crashes the game if you shoot at the doors locks
2. The mod crashes the game if you looking at forklift on Alesso heist. Firstly - _last_unit doesn't seems to always have character_damage And secondly - _HEALTH_INIT and _last_unit:base() doesn't always exists. I have no idea why, but this changes are fixing it.

[Vlad-00003]

@Kamikaze94, so? This is crusual update...