Fix two new compiler warnings in fuzz

[TheBlueMatt]

This fixes two trivial compiler warnings in fuzz that point to
broken usage of explicit panic!()s.

[codecov]

Codecov Report

Merging #859 (d015ff2) into main (6fcac8b) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #859      +/-   ##
==========================================
+ Coverage   90.60%   90.62%   +0.02%     
==========================================
  Files          51       51              
  Lines       27193    27193              
==========================================
+ Hits        24637    24644       +7     
+ Misses       2556     2549       -7     

Impacted Files	Coverage Δ
lightning/src/ln/functional_tests.rs	97.02% <0.00%> (+0.12%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6fcac8b...d015ff2. Read the comment docs.