child_process: fix parsing messages with splitted length field

[MGorkov]

Fixes: #55834

[juanarbol]

Hey! Amazing, nice patch, thank you.

Could you please add a test for it?

[MGorkov]

@juanarbol I've written a test where the child process sends messages to the main, and the third message is split into parts so that the header with the length of the message is split too, which leads to the problem in the current version but my patch solves it

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/64036/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.53%. Comparing base (61e4ad5) to head (a7ef03a).
Report is 104 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #56106      +/-   ##
==========================================
+ Coverage   88.01%   88.53%   +0.51%     
==========================================
  Files         656      657       +1     
  Lines      188982   190208    +1226     
  Branches    35997    36527     +530     
==========================================
+ Hits       166330   168396    +2066     
+ Misses      15825    14995     -830     
+ Partials     6827     6817      -10     

Files with missing lines	Coverage Δ
lib/internal/child_process/serialization.js	95.85% <100.00%> (+0.12%)	⬆️

... and 140 files with indirect coverage changes

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/64056/

[juanarbol]

Thanks!