Avoid crash in PlacementAmongFloats (#30235)

PlacementAmongFloats was assuming that there would always be a FloatBand
at a position smaller than or equal to the given ceiling, but this was
not the case for negative ceilings.

The reason is that the FloatContext initialized the FloatBandTree with
a band at 0, and another at +∞.

This patch changes the initial bands to −∞ and +∞. This seems more
consistent and matches the expectation of PlacementAmongFloats.

components/layout_2020/flow/float.rs

@@ -307,7 +307,7 @@ impl FloatContext {
     pub fn new(max_inline_size: Length) -> Self {
         let mut bands = FloatBandTree::new();
         bands = bands.insert(FloatBand {
-            top: Length::zero(),
+            top: Length::new(-f32::INFINITY),
             left: None,
             right: None,
         });