fix: crash with source maps when the parallel option is false

[alexander-akait]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

fixes #51

Breaking Changes

No

Additional Info

No

[codecov]

Codecov Report

Merging #53 (385949b) into master (bdb3f52) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   97.22%   97.23%   +0.01%     
==========================================
  Files           5        5              
  Lines         252      253       +1     
  Branches       95       96       +1     
==========================================
+ Hits          245      246       +1     
  Misses          6        6              
  Partials        1        1              

Impacted Files	Coverage Δ
src/minify.js	95.83% <100.00%> (+0.18%)	⬆️

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