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Don't free a TargetEntry that Qiskit has already taken (#84)

* Don't free a TargetEntry that Qiskit has already taken

`qk_target_add_instruction` cleared the wrapper's pointer only after the C call
returned normally, so a call that raised left the pointer set.  Qiskit takes
ownership of a valid entry whether or not the instruction is successfully added,
so the entry had already been freed, and our finalizer freed it a second time.

Reaching this needs no unusual code -- build a target, add the same gate twice,
handle the error, carry on:

    try
        qk_target_add_instruction(target, duplicate)
    catch
    end

The second free aborts the process with `free(): invalid size` once the
finalizer runs, and in a loop it instead segfaults during finalization at exit,
after the user's code has finished and with nothing to point at the cause.
Neither is catchable from Julia.

Clear the pointer in a `finally` block so it is cleared on both paths.

That Qiskit consumes the entry unconditionally is confirmed by the maintainers
in https://github.com/Qiskit/qiskit/issues/17051 -- an `AlignmentError` return
is the only case that does not take ownership, and in that case the pointer was
never a valid entry to begin with, so there is nothing to free.

Assisted-by: Claude Opus 5

* Prune a comment

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