fix packing of 1-RTT probe packets

[marten-seemann]

Fixes #2878.

[codecov]

Codecov Report

Merging #2882 (47c3cd8) into master (ad8367c) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2882      +/-   ##
==========================================
- Coverage   85.77%   85.77%   -0.00%     
==========================================
  Files         134      134              
  Lines        9188     9187       -1     
==========================================
- Hits         7881     7880       -1     
  Misses        959      959              
  Partials      348      348              

Impacted Files	Coverage Δ
packet_packer.go	84.42% <100.00%> (-0.03%)	⬇️

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