Fix nth_root_exact panic on negative even roots

[sylvestre]

Closes: #25

[codspeed-hq]

Merging this PR will degrade performance by 93.2%

⚡ 1 improved benchmark
❌ 3 regressed benchmarks
✅ 25 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	glass_pumpkin	33.9 ms	26.1 ms	+29.63%
❌	Simulation	num-primes	22 ms	54.6 ms	-59.67%
❌	Simulation	glass_pumpkin	3.2 ms	6.9 ms	-54.45%
❌	Simulation	num-primes	759.5 ms	11,165.9 ms	-93.2%

---

_{Comparing sylvestre:bug-25 (d9f280a) with main (75e7ea8)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.97%. Comparing base (3f5db7d) to head (d9f280a).
⚠️ Report is 22 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   78.64%   84.97%   +6.32%     
==========================================
  Files          13       14       +1     
  Lines        2585     3460     +875     
  Branches      230      240      +10     
==========================================
+ Hits         2033     2940     +907     
+ Misses        552      519      -33     
- Partials        0        1       +1     

Flag	Coverage Δ
macos_latest	84.94% <100.00%> (+7.30%)	⬆️
ubuntu_latest	84.50% <100.00%> (+6.32%)	⬆️
windows_latest	17.63% <14.03%> (+2.92%)	⬆️

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