Fix GH-17223: Memory leak in libxml encoding handling

This was a bug in both libxml and PHP.
We follow up with the same change as done in GNOME/libxml@b3871dd138.

Changing away

Fix GH-17223: Memory leak in libxml encoding handling

This was a bug in both libxml and PHP.
We follow up with the same change as done in GNOME/libxml@b3871dd138.

Changing away from `xmlOutputBufferCreateFilenameDefault` is not
possible yet because this is a stable branch and would break BC.

Closes GH-17254.

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