Add explicit assert to ensure thread_shared_qb won't expand

[enjoy-binbin]

Although this won't happen now, adding this statement explicitly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.40%. Comparing base (8045994) to head (0fefdc0).
Report is 5 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #938      +/-   ##
============================================
- Coverage     70.64%   70.40%   -0.24%     
============================================
  Files           112      112              
  Lines         61516    61518       +2     
============================================
- Hits          43459    43314     -145     
- Misses        18057    18204     +147     

Files	Coverage Δ
src/networking.c	88.82% <100.00%> (+0.05%)	⬆️

... and 12 files with indirect coverage changes