[PR #10088/29c3ca93 backport][3.12] Avoid calling len on the same data in the stream reader twice

[patchback]

This is a backport of PR #10088 as merged into master (29c3ca9).

This is trivial but it is noticeable in the profile.

before

after

[codspeed-hq]

CodSpeed Performance Report

Merging #10091 will not alter performance

_{Comparing patchback/backports/3.12/29c3ca93a4d1793c60d33038151228962f8b6f78/pr-10088 (cdc6ea9) with 3.12 (bc6a33e)}

Summary

✅ 44 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.06%. Comparing base (bc6a33e) to head (cdc6ea9).
Report is 1 commits behind head on 3.12.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             3.12   #10091   +/-   ##
=======================================
  Coverage   98.06%   98.06%           
=======================================
  Files         122      122           
  Lines       37051    37052    +1     
  Branches     4453     4453           
=======================================
+ Hits        36334    36335    +1     
  Misses        540      540           
  Partials      177      177           

Flag	Coverage Δ
CI-GHA	97.95% <100.00%> (+<0.01%)	⬆️
OS-Linux	97.66% <100.00%> (+<0.01%)	⬆️
OS-Windows	94.57% <100.00%> (+<0.01%)	⬆️
OS-macOS	96.78% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	96.63% <100.00%> (+<0.01%)	⬆️
Py-3.10.15	97.17% <100.00%> (-0.05%)	⬇️
Py-3.11.10	97.24% <100.00%> (-0.05%)	⬇️
Py-3.11.9	96.70% <100.00%> (-0.01%)	⬇️
Py-3.12.7	97.72% <100.00%> (+<0.01%)	⬆️
Py-3.13.0	97.71% <100.00%> (-0.01%)	⬇️
Py-3.9.13	96.56% <100.00%> (-0.01%)	⬇️
Py-3.9.20	97.14% <100.00%> (+<0.01%)	⬆️
Py-pypy7.3.16	96.69% <100.00%> (-0.02%)	⬇️
VM-macos	96.78% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	97.66% <100.00%> (+<0.01%)	⬆️
VM-windows	94.57% <100.00%> (+<0.01%)	⬆️

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