Fix bad memory accounting for sds when no malloc_size available

[ranshid]

Issue Introduced by #453.
When we check the SDS _TYPE_5 allocation size we mistakenly used zmalloc_size which DOES take the PREFIX size into account when no malloc_size support.
Later when we free we add the PREFIX_SIZE again which leads to negative memory accounting on some tests.
Example test failure: https://github.com/valkey-io/valkey/actions/runs/9654170962/job/26627901497

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.24%. Comparing base (4d3d6c0) to head (45ed952).

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #694      +/-   ##
============================================
+ Coverage     70.21%   70.24%   +0.02%     
============================================
  Files           110      110              
  Lines         60100    60100              
============================================
+ Hits          42202    42215      +13     
+ Misses        17898    17885      -13     

Files	Coverage Δ
src/sds.c	85.86% <100.00%> (ø)

... and 9 files with indirect coverage changes