This historically was used to guard against recursive faults but
later refactorings have made this variable somewhat obsolete. The code
that it still protects is not the "meat" of trap handling. Instead the
`jmp_buf_if_trap` is changed to be more like "take" so once a "take"
succeeds it won't be able to recursively call any more "meat".
Overall this shouldn't affect anything, it's just a small internal
cleanup.
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