Plug memory leak

[JKingweb]

This bug existed in Picofeed originally and had been reported by me, but had not been acted upon before the repository was archived and, ultimately, deleted.

When fetching HTML documents Picofeed would buffer libxml's error reports, but then would never clear the buffer. Any subsequent use of document loading with the DOM class would thereafter also buffer errors, needlessly consuming memory.

[aaronpk]

The function that stores the errors is static, so $errors needs to be defined static and written with self::$errors[] instead of $this->errors[]. Here is a diff for what needs to change in this PR: aaronpk/picofeed@b1f4448

[JKingweb]

I'm embarrassed I missed that. Thanks.

[coveralls]

Pull Request Test Coverage Report for Build 69

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 95.474%

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Totals
Change from base Build 63:	0.002%
Covered Lines:	1772
Relevant Lines:	1856

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💛 - Coveralls

[nicolus]

Hi there ! Thanks for the PR.
Sorry I'm a bit behind, I'll take some time to merge everything this week.