Fix compile-time warnings in fuzzing

[TheBlueMatt]

Introduced in two recent PRs.

[codecov-commenter]

Codecov Report

Base: 90.80% // Head: 90.77% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (f1d70be) compared to base (71f4749).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1720      +/-   ##
==========================================
- Coverage   90.80%   90.77%   -0.03%     
==========================================
  Files          86       86              
  Lines       46670    46670              
  Branches    46670    46670              
==========================================
- Hits        42377    42367      -10     
- Misses       4293     4303      +10     

Impacted Files	Coverage Δ
lightning/src/chain/channelmonitor.rs	91.20% <ø> (ø)
lightning-net-tokio/src/lib.rs	76.73% <0.00%> (-0.31%)	⬇️
lightning/src/chain/onchaintx.rs	95.40% <0.00%> (-0.23%)	⬇️
lightning/src/ln/functional_tests.rs	96.87% <0.00%> (-0.14%)	⬇️
lightning-invoice/src/utils.rs	96.77% <0.00%> (+0.14%)	⬆️

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