avoid allocating a shortHeaderPacket struct in the PackPacket path

[marten-seemann]

Depends on #3646.

I'm not 100% sure why this helps, but it makes the appendShortHeaderPacket code path disappear from the alloc profile. I assume that this helps the compiler do escape analysis?

[codecov]

Codecov Report

Base: 85.06% // Head: 85.06% // No change to project coverage 👍

Coverage data is based on head (c24fbb0) compared to base (c24fbb0).
Patch has no changes to coverable lines.

❗ Current head c24fbb0 differs from pull request most recent head e10af24. Consider uploading reports for the commit e10af24 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3647   +/-   ##
=======================================
  Coverage   85.06%   85.06%           
=======================================
  Files         140      140           
  Lines       10338    10338           
=======================================
  Hits         8793     8793           
  Misses       1140     1140           
  Partials      405      405           

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