Add guards to verify that the unit instance still exists

[Garanas]

Description of the proposed changes

Fixes a rather invisible bug from #6263 .

The logging when the pcall fails is disabled at the moment. Luckily, the errors are caught and registered by the c-debugger. The tables with unit references in them are cleaned up automagically by the garbage collector, but there can be a few ticks in between the unit dying and the unit being collected. In those few ticks it can generate various errors.

@c:\programdata\faforever\replaydata\gamedata\lua.nx2\lua\aibrains\components\storagemanagerbraincomponent.lua 179
CObject:.?AVCAiBrain@Moho@@, CFunc, "EconMidMassStore", "EconMidEnergyStore"
@c:\programdata\faforever\replaydata\gamedata\lua.nx2\lua\aibrains\components\storagemanagerbraincomponent.lua 187
CObject:.?AVCAiBrain@Moho@@, "EconMidEnergyStore", CFunc, Table, "8388651", Table, Func:@c:\programdata\faforever\replaydata\gamedata\lua.nx2\lua\sim\units\factoryunit.lua 510
@c:\programdata\faforever\replaydata\gamedata\lua.nx2\lua\aibrains\components\storagemanagerbraincomponent.lua 169
@CFunc: 0x0090F5B0
Table, "EconMidEnergyStore"
@c:\programdata\faforever\replaydata\gamedata\lua.nx2\lua\sim\units\factoryunit.lua 514
Table, "Green", "Red", CObject:.?AVCAiBrain@Moho@@
@c:\programdata\faforever\replaydata\gamedata\lua.nx2\lua\sim\units\components\blinkinglightsunitcomponent.lua 56
Table, UData:1B2F8F88, "...sim\units\components\blinkinglightsunitcomponent.lua(56): Game object has been destroyed
stack traceback:
	[C]: in function `GetNumBuildOrders'
	...sim\units\components\blinkinglightsunitcomponent.lua(56): in function `ChangeBlinkingLights'
	...ydata\gamedata\lua.nx2\lua\sim\units\factoryunit.lua(514): in function <...ydata\gamedata\lua.nx2\lua\sim\units\factoryunit.lua:510>
	[C]: in function `pcall'
	...aibrains\components\storagemanagerbraincomponent.lua(169): in function `ApplyEnergyStorageState'
	...aibrains\components\storagemanagerbraincomponent.lua(187): in function <...aibrains\components\storagemanagerbraincomponent.lua:179>"

Checklist

• Changes are annotated, including comments where useful
• Changes are documented in the changelog for the next game version