SVG: Crash when referencing (use) non-existing elements

[pbregener]

Rendering a SVG that uses non-existing elements results in a crash.

This has been a problem in all v53 betas, including current master . The cairo-based version simply ignored those use statements.

There are various potentially trivial fixes, let me know if you'd prefer a PR over fixing this yourself @liZe :)

Example SVG:

<svg viewBox="0 0 16 10" xmlns="http://www.w3.org/2000/svg">
  <circle id="valid" cx="5" cy="5" r="4" stroke="green"/>
  <use href="#valid" x="6" fill="green"/>
  <use href="#notvalid" x="7" fill="red"/>
</svg>

Short backtrace:

  File "[...]\weasyprint\svg\defs.py", line 30, in use
    tree = svg.tree.get_child(parsed_url.fragment).copy()
AttributeError: 'NoneType' object has no attribute 'copy'

WeasyPrint/weasyprint/svg/defs.py

Line 30 in 56910a8

tree = svg.tree.get_child(parsed_url.fragment).copy()