Add guards in fire beetle target tracker thread

[lL1l1]

Description of the proposed changes

Solves errors in replay 27759573 (faf beta version bf1e5fe)

warning: Error running lua script: ...\gamedata\units.nx4\units\xrl0302\xrl0302_script.lua(133): Game object has been destroyed
         stack traceback:
         	[C]: in function `SetDestUnit'
         	...\gamedata\units.nx4\units\xrl0302\xrl0302_script.lua(133): in function <...\gamedata\units.nx4\units\xrl0302\xrl0302_script.lua:107>

warning: Error running lua script: ...\gamedata\units.nx4\units\xrl0302\xrl0302_script.lua(124): Game object has been destroyed
         stack traceback:
         	[C]: in function `SetEnabled'
         	...\gamedata\units.nx4\units\xrl0302\xrl0302_script.lua(124): in function <...\gamedata\units.nx4\units\xrl0302\xrl0302_script.lua:107>

Testing done on the proposed changes

Additional context

Checklist

• Changes are annotated, including comments where useful
• Changes are documented in a changelog snippet according to the guidelines.
• Request 2-3 reviewers from the list of reviewers and their areas of knowledge.

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