Output: optimize fix_malformed_script_link_tags() regex

[swissspidy]

Context

See #12158.

This error with a blank screen can happen when the preg_replace_callback() call in HTML::fix_malformed_script_link_tags() errored and thus an empty content was returned.

In my testing this all happened because

1. The existing regex was very inefficient and had catastrophic backtracking
2. We simply overrode $content even if there was an error

Summary

This PR optimizes the preg_replace_callback usage in fix_malformed_script_link_tags() in two ways:

1. Use optimized regex that is simpler to read (using a back reference) and has better performance
2. Return original $content if there was an error with the regex replacement

Relevant Technical Choices

Removed some dead code

To-do

N/A

User-facing changes

N/A

Testing Instructions

• This is a non-user-facing change and requires no QA

This PR can be tested by following these steps:

Reviews

Does this PR have a security-related impact?

No

Does this PR change what data or activity we track or use?

No

Does this PR have a legal-related impact?

No

Checklist

• This PR addresses an existing issue and I have linked this PR to it in ZenHub
• I have tested this code to the best of my abilities
• I have verified accessibility to the best of my abilities (docs)
• I have verified i18n and l10n (translation, right-to-left layout) to the best of my abilities
• This code is covered by automated tests (unit, integration, and/or e2e) to verify it works as intended (docs)
• I have added documentation where necessary
• I have added a matching Type: XYZ label to the PR

---

Fixes #12158

[googleforcreators-bot]

Plugin builds for 839bc90 are ready 🛎️!

• Download development build
• Download production build