fix(http): NPE in the `http.Reset` method

[rustatian]

Reason for This PR

ref: https://github.com/spiral/roadrunner-plugins/issues/155

Description of Changes

• Check workers pool before accessing it.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.] [Reviewer TODO: Verify that these criteria are met. Request changes if not]

• [x] All commits in this PR are signed (git commit -s).
• [x] The reason for this PR is clearly provided (issue no. or explanation).
• [x] The description of changes is clear and encompassing.
• [x] Any required documentation changes (code and docs) are included in this PR.
• [x] Any user-facing changes are mentioned in CHANGELOG.md.
• [x] All added/changed functionality is tested.

[codecov[bot]]

Codecov Report

Merging #147 (111730a) into master (fb1b783) will not change coverage. The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #147   +/-   ##
=======================================
  Coverage   43.56%   43.56%           
=======================================
  Files          12       12           
  Lines         567      567           
=======================================
  Hits          247      247           
  Misses        309      309           
  Partials       11       11           

Impacted Files	Coverage Δ
internal/cli/workers/command.go	19.14% <0.00%> (ø)
internal/cli/workers/render.go	0.00% <0.00%> (ø)

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