Fix for liquid handling during butchery after interruption

[ghost]

Summary

SUMMARY: Bugfixes "Fix for liquid handling during butchery after interruption"

Purpose of change

Fixes #35514

Describe the solution

The activity was cancelled halfway through the butcher_finish function, due to the liquid handling calling cancel_activity when interrupted, this clears the activity targets etc.
Then the rest of the function finishes, and tries to pop_back() on empty activity targets and remove the item that was a reference to activity target.

Ive made the target a non-reference, and checked for targets being non-empty before pop_back() as these things are changed during the function.

Describe alternatives you've considered

N/A

Testing

modified the code to spawn a zombie during liquid handling in butcher_finish() , this forced an interruption every time, caused a crash every time.
Now no crash.

Additional context

N/A