Fix #79971: special character is breaking the path in xml function

The libxml based XML functions accepting a filename actually accept
URIs with possibly percent-encoded characters. Per

Fix #79971: special character is breaking the path in xml function

The libxml based XML functions accepting a filename actually accept
URIs with possibly percent-encoded characters. Percent-encoded NUL
bytes lead to truncation, like non-encoded NUL bytes would. We catch
those, and let the functions fail with a respective warning.

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