fixed division by zero

[lbw114007]

fixed error:
{"level":"error","ts":1627470361.411179,"caller":"master/recommend.go:481","msg":"failed to insert measurement","error":"Error 1264: Out of range value for column 'value' at row 1","errorVerbose":"Error 1264: Out of range value for column 'value' at row 1\nE:/GoProjects/src/github.com/zhenghaoz/gorse/storage/data/sql.go:156: ","stacktrace":"github.com/zhenghaoz/gorse/master.(*Master).fitClickModel\n\tE:/GoProjects/src/github.com/zhenghaoz/gorse/master/recommend.go:481\ngithub.com/zhenghaoz/gorse/master.(*Master).FitLoop\n\tE:/GoProjects/src/github.com/zhenghaoz/gorse/master/master.go:267"}

[codecov-commenter]

Codecov Report

Merging #221 (dbc1dc7) into master (d9b3f16) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #221      +/-   ##
==========================================
- Coverage   63.93%   63.85%   -0.08%     
==========================================
  Files          41       41              
  Lines        6630     6638       +8     
==========================================
  Hits         4239     4239              
- Misses       1916     1922       +6     
- Partials      475      477       +2     

Impacted Files	Coverage Δ
model/click/evaluator.go	70.58% <0.00%> (-21.72%)	⬇️

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