Fix a deadlock in the lazy join.

[joka921]

No description provided.

[sparql-conformance]

Conformance check passed ✅

No test result changes.

Details: https://qlever.cs.uni-freiburg.de/sparql-conformance-ui?cur=d2c8596d081d78ba2a57e800829fb63c42c213a7&prev=621d39d82c698a9ea814b68021e2f3eeac5e22bf

[codecov]

Codecov Report

Attention: Patch coverage is 90.74074% with 5 lines in your changes missing coverage. Please review.

Project coverage is 89.37%. Comparing base (add092a) to head (d2c8596).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/engine/Join.cpp	90.74%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1649   +/-   ##
=======================================
  Coverage   89.37%   89.37%           
=======================================
  Files         375      375           
  Lines       36107    36128   +21     
  Branches     4077     4078    +1     
=======================================
+ Hits        32272    32291   +19     
  Misses       2518     2518           
- Partials     1317     1319    +2     

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[sonarcloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[RobinTF]

Closing in favour of #1650