Avoid panic when k for k-neighbor search is zero

[msk]

Fixes a panic in BallTree::query when k = 0.

[codecov]

Codecov Report

Merging #33 into master will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   92.63%   92.74%   +0.10%     
==========================================
  Files           3        3              
  Lines         421      427       +6     
==========================================
+ Hits          390      396       +6     
  Misses         31       31              

Impacted Files	Coverage Δ
src/ball_tree.rs	93.82% <100.00%> (+0.11%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 77a89cf...2391ed0. Read the comment docs.