Fix missing cost initialization

[jnidzwetzki]

The Sanitizer build found a missing memory initialization of the aggregation costs. This PR adds the missing initialization.

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Disable-check: force-changelog-file

Failing CI run: https://github.com/timescale/timescaledb/actions/runs/6192216103/job/16824687929
Fixed CI run: https://github.com/timescale/timescaledb/actions/runs/6197194660/job/16825298034

[codecov]

Codecov Report

Merging #6074 (6c34b00) into main (7fbaf2a) will decrease coverage by 0.07%.
Report is 2 commits behind head on main.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #6074      +/-   ##
==========================================
- Coverage   81.57%   81.51%   -0.07%     
==========================================
  Files         246      246              
  Lines       56535    56535              
  Branches    12519    12518       -1     
==========================================
- Hits        46120    46085      -35     
- Misses       8071     8082      +11     
- Partials     2344     2368      +24     

Files Changed	Coverage Δ
src/planner/partialize.c	94.28% <0.00%> (-0.46%)	⬇️

... and 18 files with indirect coverage changes

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