Fix for ZeroDivisionError when verbose_eval equals to 0.

[oleg-panichev]

This pull request addresses an issue when verbose_eval parameter set to zero leads to ZeroDivisionError: integer division or modulo by zero.

[codecov-io]

Codecov Report

Merging #3115 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #3115   +/-   ##
=========================================
  Coverage     43.79%   43.79%           
  Complexity      228      228           
=========================================
  Files           159      159           
  Lines         12507    12507           
  Branches        466      466           
=========================================
  Hits           5478     5478           
  Misses         6837     6837           
  Partials        192      192

Impacted Files	Coverage Δ	Complexity Δ
python-package/xgboost/callback.py	87.8% <100%> (ø)	0 <0> (ø)	⬇️

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