Allow thereafter in sampler to be zero

[gunnihinn]

When creating a new Sampler, there is nothing stopping us from passing in 0 as a value for thereafter. One might expect this to be legal -- as the documentation doesn't say thereafter should be greater than zero and no error is returned when passing in this configuration -- and to mean "drop all messages after the first n". What actually happens is a

panic: runtime error: integer divide by zero

when we try to log a message.

This patch avoids the division by zero and enforces the behavior described above.

This fixes issue 632.

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[codecov[bot]]

Codecov Report

Merging #633 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #633   +/-   ##
=======================================
  Coverage   97.38%   97.38%           
=======================================
  Files          40       40           
  Lines        2102     2102           
=======================================
  Hits         2047     2047           
  Misses         47       47           
  Partials        8        8

Impacted Files	Coverage Δ
zapcore/sampler.go	100% <100%> (ø)	:arrow_up:

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[gunnihinn]

since using 0 means we're disabling sampling.

Did I miss where we can choose to log only the first first messages?