detect/stream_size: fix prefiltering registration

[regit]

Prefiltering registration was using an invalid signature index.

• I have read the contributing guide lines at
  https://docs.suricata.io/en/latest/devguide/codebase/contributing/contribution-process.html
• I have signed the Open Information Security Foundation contribution agreement at
  https://suricata.io/about/contribution-agreement/ (note: this is only required once)
• I have updated the user guide (in doc/userguide/) to reflect the changes made (if applicable)

Link to redmine ticket: https://redmine.openinfosecfoundation.org/issues/6551

Describe changes:

• Fix prefilter registration

[codecov]

Codecov Report

Merging #9830 (5ca0576) into master (d2b25af) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9830      +/-   ##
==========================================
- Coverage   82.42%   82.42%   -0.01%     
==========================================
  Files         972      972              
  Lines      273929   273929              
==========================================
- Hits       225788   225784       -4     
- Misses      48141    48145       +4     

Flag	Coverage Δ
fuzzcorpus	64.29% <0.00%> (-0.01%)	⬇️
suricata-verify	61.09% <0.00%> (+<0.01%)	⬆️
unittests	62.88% <0.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[suricata-qa]

Information: QA ran without warnings.

Pipeline 16630

[victorjulien]

Merged in #9831, thanks!