unionRect(const Vector<LayoutRect>&) behaves incorrectly with empty r…

…ects

https://bugs.webkit.org/show_bug.cgi?id=259674
rdar://113183595

Reviewed by Alan Baradlay.

unionRect(const Vector<LayoutRect>&) behaves incorrectly if the input vector contains
a single zero-size rect with a non-zero location; it will union with the empty rect,
and therefore return the empty rect.

This is not known to change any behavior, but a future change in bug 259672
will exercise this code.

* Source/WebCore/platform/graphics/LayoutRect.cpp:
(WebCore::unionRect):

Canonical link: https://commits.webkit.org/266479@main

Source/WebCore/platform/graphics/LayoutRect.cpp

@@ -180,10 +180,12 @@ void LayoutRect::expandToInfiniteX()
 
 LayoutRect unionRect(const Vector<LayoutRect>& rects)
 {
-    LayoutRect result;
+    if (rects.isEmpty())
+        return { };
 
+    LayoutRect result = rects[0];
     size_t count = rects.size();
-    for (size_t i = 0; i < count; ++i)
+    for (size_t i = 1; i < count; ++i)
         result.unite(rects[i]);
 
     return result;