Fix an invalid read.

[drwells]

If num_results == 0 then dynamic_idx is empty, so dynamic_idx[0] does not exist. I caught this by building nanoflann with -D_GLIBCXX_ASSERTIONS.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.85%. Comparing base (92911c0) to head (beb32c4).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #282   +/-   ##
=======================================
  Coverage   83.85%   83.85%           
=======================================
  Files           1        1           
  Lines         675      675           
  Branches      127      127           
=======================================
  Hits          566      566           
  Misses        109      109           

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[jlblancoc]

Thanks!!