fix golangci-lint error G602 in caddyhttp

[cdenicola]

Commit ddec183 was failing CI due to lint error:

  Running [/Users/runner/golangci-lint-2.6.0-darwin-arm64/golangci-lint run  --timeout 10m] in [/Users/runner/work/caddy/caddy] ...
  Error: modules/caddyhttp/reverseproxy/fastcgi/writer.go:115:3: G602: slice index out of range (gosec)
  	b[0] = byte(size)
  	 ^

This linter-failing line is from an old PR. Looking at older CI logs, it appears a successful cache of this file's lint recently expired, causing the error to appear

Code chunk is safe because function is only used when passing in a buffer of size >=4, but the linter is unable to guarantee this

I tested code changes to make the linter pass, but the error remained unless an explicit size check was added. It does not feel necessary to check the size of buffer every time in encodeSize when we guaranteed in caller that buffer is big enough. As such commenting to disable the linter error felt like the least invasive solution. Happy to change to whatever fits best the project's, I found this project recently, have enjoyed using it on my own, and wanted to make a small first PR_

Assistance Disclosure

Used AI to test code changes that would disable linter. No AI code (or really any code) entered this PR

[CLAassistant]

All committers have signed the CLA.

[francislavoie]

Thanks, @mholt fixed it in his branch with https://github.com/caddyserver/caddy/pull/7324/files#diff-384ea25b96b47d84c242c7fe3341847cb468d867fafdea97d0920b605321a35dR115 but it didn't get merged yet. I think we should change it to match how Matt did it in his so it avoids the merge conflict

[cdenicola]

Thanks @francislavoie. I updated the PR to match #7324 change to prevent later merge conflicts