calculate version negotiation packet size to avoid bytes.Buffer growi…

[mw-jn]

Fixes #2541.

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[codecov-io]

Codecov Report

Merging #2542 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2542   +/-   ##
=======================================
  Coverage   86.15%   86.15%           
=======================================
  Files         122      122           
  Lines        9542     9542           
=======================================
  Hits         8220     8220           
  Misses        984      984           
  Partials      338      338           

Impacted Files	Coverage Δ
internal/wire/version_negotiation.go	100.00% <100.00%> (ø)

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[mw-jn]

@googlebot I signed it!

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We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[mw-jn]

@googlebot I fixed it.

[googlebot]

CLAs look good, thanks!

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[marten-seemann]

Thank you!