fix(psbt): sanity check psbt before signing

[sdmg15]

Description

This PR attempts to sanity check the PSBT before signing. It closes audit issue #52

Notes to the reviewers

Changelog notice

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 80.39%. Comparing base (4b612f5) to head (f0aca23).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #486      +/-   ##
==========================================
+ Coverage   80.30%   80.39%   +0.09%     
==========================================
  Files          24       24              
  Lines        5417     5432      +15     
  Branches      245      246       +1     
==========================================
+ Hits         4350     4367      +17     
+ Misses        989      988       -1     
+ Partials       78       77       -1     

Flag	Coverage Δ
rust	80.39% <100.00%> (+0.09%)	⬆️

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