Check for undefined worker.stdio

[mistic]

Closes #2 by adding a proper check for a situation where the this.worker.stdio could become undefined due to the OS reach the max open files limit.

/cc @evilebottnawi

[mistic]

The CI fail on this PR is solved by #44

[alexander-akait]

@mistic feel free to rebase

[codecov]

Codecov Report

Merging #45 into master will increase coverage by 12.67%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master      #45       +/-   ##
===========================================
+ Coverage       0%   12.67%   +12.67%     
===========================================
  Files           7        7               
  Lines         372      355       -17     
  Branches       66       61        -5     
===========================================
+ Hits            0       45       +45     
+ Misses        325      272       -53     
+ Partials       47       38        -9

Impacted Files	Coverage Δ
src/WorkerPool.js	20.95% <100%> (+20.95%)	⬆️
src/WorkerError.js	6.25% <0%> (+6.25%)	⬆️
src/readBuffer.js	37.5% <0%> (+37.5%)	⬆️

---

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 e07fe0e...82a0a85. Read the comment docs.

[alexander-akait]

Will be great to add tests, example tests https://github.com/webpack-contrib/css-loader/blob/master/test/loader.test.js, you just need mock stdio

[mistic]

@evilebottnawi done!