3.16 - validate image source before saving it into the DB

[wordpressfan]

Description

Fixes #6599

Documentation

User documentation

Explain how this code impacts users.

Technical documentation

Explain how this code works. Diagram & drawings are welcomed.

Type of change

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• [ ] New feature (non-breaking change which adds functionality).
• [ ] Bug fix (non-breaking change which fixes an issue).
• [ ] Enhancement (non-breaking change which improves an existing functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as before).

New dependencies

List any new dependencies that are required for this change.

Risks

List possible performance & security issues or risks, and explain how they have been mitigated.

Checklists

Feature validation

• [ ] I validated all the Acceptance Criteria. If possible, provide sreenshots or videos.
• [ ] I triggered all changed lines of code at least once without new errors/warnings/notices.
• [ ] I implemented built-in tests to cover the new/changed code.

Documentation

• [ ] I prepared the user documentation for the feature/enhancement and shared it in the PR or the GitHub issue.
• [ ] The user documentation covers new/changed entry points (endpoints, WP hooks, configuration files, ...).
• [ ] I prepared the technical documentation if needed, and shared it in the PR or the GitHub issue.

Code style

• [ ] I wrote self-explanatory code about what it does.
• [ ] I wrote comments to explain why it does it.
• [ ] I named variables and functions explicitely.
• [ ] I protected entry points against unexpected inputs.
• [ ] I did not introduce unecessary complexity.
• [ ] I listed the introduced external dependencies explicitely on the PR.
• [ ] I validated the repo-specific guidelines from CONTRIBUTING.md.

Observability

• [ ] I handled errors when needed.
• [ ] I wrote user-facing messages that are understandable and provide actionable feedbacks.
• [ ] I prepared ways to observe the implemented system (logs, data, etc.).

Risks

• [ ] I explicitely mentioned performance risks in the PR.
• [ ] I explicitely mentioned security risks in the PR.