Check if instigator is destroyed in Fatboy DeathThread

[lL1l1]

Issue

Since the instigator is used in a thread, and threads start 1 tick later, it can be passed from OnKilled and then get destroyed.
Replay: https://replay.faforever.com/24087043 Right side fatboy for south team at 29:00 errors out when dying
Error:

warning: Error running lua script: ...\gamedata\units.nx2\units\uel0401\uel0401_script.lua(263): Game object has been destroyed
         stack traceback:
         	[C]: in function `GetPositionXYZ'
         	...\gamedata\units.nx2\units\uel0401\uel0401_script.lua(263): in function <...\gamedata\units.nx2\units\uel0401\uel0401_script.lua:253>

Description of the proposed changes

• Check if the instigator was destroyed before calling GetPositionXYZ on it. I didn't opt for using Unit.Dead because this code won't run often and .Dead is true after dying, which is before C-object destruction.

Testing done on the proposed changes

• Running the replay from deploy/faf (805bdd3) with the fix commit cherry picked on top causes the fatboy to successfully die
• Checked that no other DeathThreads use the instigator value.

Checklist

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