servo: Merge #19652 - Fix division by zero in gradient stop calculation (from pyfisch:issue18435); r=emilio

Check if total_length is zero and return 0.0 instead
of NaN in this case.

Closes #18435

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- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #18435 (github issue number if applicable).

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Source-Repo: https://github.com/servo/servo
Source-Revision: 1ce6d8ea2d2bbb4d6f095ac62514eca942599e83

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : e30331dc8bdbe4af4687be81956e31850d4e5613

servo/components/layout/display_list_builder.rs

@@ -798,6 +798,7 @@ fn convert_gradient_stops(gradient_items: &[GradientItem],
                 position_to_offset(position, total_length)
             }
         };
+        assert!(offset.is_finite());
         stops.push(GradientStop {
             offset: offset,
             color: stop.color.to_gfx_color()
@@ -3270,6 +3271,9 @@ struct StopRun {
 }
 
 fn position_to_offset(position: LengthOrPercentage, total_length: Au) -> f32 {
+    if total_length == Au(0) {
+        return 0.0
+    }
     match position {
         LengthOrPercentage::Length(l) => l.to_i32_au() as f32 / total_length.0 as f32,
         LengthOrPercentage::Percentage(percentage) => percentage.0 as f32,