LibWeb: Make stretch-fit return 0 when the available size is indefinite

The spec says the result of this algorithm is undefined in such cases,
and it appears that other engines yield a zero size.

More importantly, this prevents us from leaking a non-finite value into
the layout tree.

Userland/Libraries/LibWeb/Layout/FormattingContext.cpp

@@ -1436,7 +1436,11 @@ CSSPixels FormattingContext::calculate_stretch_fit_width(Box const& box, Availab
 {
     // The size a box would take if its outer size filled the available space in the given axis;
     // in other words, the stretch fit into the available space, if that is definite.
+
     // Undefined if the available space is indefinite.
+    if (!available_width.is_definite())
+        return 0;
+
     auto const& box_state = m_state.get(box);
     return available_width.to_px()
         - box_state.margin_left