fix(types): use bounds-checked access in Utxo::txout() for Foreign variant

[tnull]

Description

Replace unchecked array indexing of prev_tx.output[outpoint.vout] with .get().expect() to provide a descriptive panic message instead of an opaque index-out-of-bounds if the outpoint vout is invalid.

Co-Authored-By: HAL 9000

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.46%. Comparing base (fb7681a) to head (2bd536f).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #445      +/-   ##
==========================================
+ Coverage   80.04%   80.46%   +0.42%     
==========================================
  Files          24       24              
  Lines        5336     5417      +81     
  Branches      242      242              
==========================================
+ Hits         4271     4359      +88     
+ Misses        987      980       -7     
  Partials       78       78              

Flag	Coverage Δ
rust	80.46% <100.00%> (+0.42%)	⬆️

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[luisschwab]

cACK, can you fix the clippy warning?

let psbt_input = psbt::Input {
    non_witness_utxo: Some(prev_tx),
    ..Default::default()
};

[tnull]

cACK, can you fix the clippy warning?

let psbt_input = psbt::Input {
    non_witness_utxo: Some(prev_tx),
    ..Default::default()
};

Done.

[luisschwab]

tACK 2bd536f

[ValuedMammal]

I suppose this is marginally better than before. I'm not too concerned with asserting the content of the panic message string. If anything the method should state how it panics in the documentation. Noting that it generally works when combined with add_foreign_utxo, by throwing for MissingUtxo, since we can't control how the foreign Utxo is constructed in all cases, I guess the right solution long term is to handle a missing txout using Option or Result.