avoid ArgumentOutOfRangeException when ship has 0 parts (post-explosion)

I had a ship explode, after which the KSP log was filled with tons of these:

```
ArgumentOutOfRangeException: Argument is out of range.
Parameter name: index
  at System.Collections.Generic.List`1[Part].get_Item (Int32 index) [0x00000] in <filename unknown>:0 
  at NearFutureElectrical.DischargeCapacitorUI.getTotalEc () [0x00000] in <filename unknown>:0 
  at NearFutureElectrical.DischargeCapacitorUI.FixedUpdate () [0x00000] in <filename unknown>:0 
```

After the ship exploded, there were 0 parts remaining, but `DischargeCapacitorUI` keeps trying to access `FlightGlobals.ActiveVessel.parts[0]` on every `FixedUpdate`.

I patched `getTotalEc` and `getTotalSc` so they won't continually throw exceptions in this case.