Avoid Vec::with_capacity(huge) on empty Routes

[TheBlueMatt]

In generally we consider empty Routes bogus garbage and don't always handle them super carefully, but ideally we shouldn't allocate a huge buffer just because someone passes a bogus Route to an onion-building utility method.

Reported by Jordan Mecom of Block's Security Team

Test by Claude Opus 4.6

[ldk-reviews-bot]

I've assigned @wpaulino as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[ldk-claude-review-bot]

I've reviewed the entire diff thoroughly. The fix is correct and well-placed. The payloads.len() - 1 on line 842 (usize arithmetic) would wrap to usize::MAX when payloads is empty, causing Vec::with_capacity(65 * usize::MAX) — a massive allocation attempt. The guard at the centralized construct_onion_packet_with_init_noise function covers all callers (construct_onion_packet, construct_trampoline_onion_packet, construct_onion_message_packet, and the test-only variant).

No issues found.

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.99%. Comparing base (2ebc372) to head (4c398bb).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/onion_utils.rs	92.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4556      +/-   ##
==========================================
- Coverage   87.01%   86.99%   -0.03%     
==========================================
  Files         163      163              
  Lines      108682   108706      +24     
  Branches   108682   108706      +24     
==========================================
- Hits        94572    94571       -1     
- Misses      11631    11655      +24     
- Partials     2479     2480       +1     

Flag	Coverage Δ
fuzzing	40.27% <40.00%> (+0.03%)	⬆️
tests	86.08% <92.30%> (-0.02%)	⬇️

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