perf: avoid double-allocation in rolling_apply_agg_window

[MarcoGorelli]

trying to learn a lesson from https://github.com/pola-rs/polars/pull/15420/files#r1546380674

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.13%. Comparing base (758b55a) to head (85a3d4e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15423   +/-   ##
=======================================
  Coverage   81.13%   81.13%           
=======================================
  Files        1362     1362           
  Lines      174802   174810    +8     
  Branches     2531     2531           
=======================================
+ Hits       141825   141839   +14     
+ Misses      32493    32487    -6     
  Partials      484      484           

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[ritchie46]

I think I will put a panic on the from impl and do a test run.