quic: reuse header support assertion

[efekrskl]

Minor code cleanup by using the existing assertion so they don't drift

[nodejs-github-bot]

Review requested:

• @nodejs/quic

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.49%. Comparing base (68df1db) to head (e0a7eb3).
⚠️ Report is 32 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66777      +/-   ##
==========================================
+ Coverage   90.47%   90.49%   +0.02%     
==========================================
  Files         791      797       +6     
  Lines      277225   277586     +361     
  Branches    53317    53330      +13     
==========================================
+ Hits       250817   251210     +393     
+ Misses      16811    16784      -27     
+ Partials     9597     9592       -5     

Files with missing lines	Coverage Δ
lib/internal/quic/quic.js	100.00% <100.00%> (ø)

... and 60 files with indirect coverage changes

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