polyfill: Return an error instead of panicking on API abuse.

[briansmith]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.47%. Comparing base (29b12ee) to head (a10c49d).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/polyfill/uninit_slice.rs	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2851      +/-   ##
==========================================
- Coverage   96.48%   96.47%   -0.01%     
==========================================
  Files         203      203              
  Lines       20337    20340       +3     
  Branches      518      519       +1     
==========================================
+ Hits        19623    19624       +1     
- Misses        595      596       +1     
- Partials      119      120       +1     

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