Panic with a mask with objectBoundingBox on an empty group
Similar to #292 (closed) - the masking code tries to unwrap()
a nonexistent bbox:
<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN"
"http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg viewBox="0 0 640 480" version="1.1" xmlns="http://www.w3.org/2000/svg">
<defs>
<mask id="foo" clipPathUnits="objectBoundingBox">
<rect x="0" y="0" width="10" height="10"/>
</mask>
</defs>
<g mask="url(#foo)"/>
</svg>
The master branch panics, but 2.42 doesn't - but it is still generating an all-zeros matrix in that case. We should catch that condition early and return.