Guard against zero item extent for carousel (#163310)

This PR fixes a bug where the `CarouselView` would throw an error if the
itemExtent is zero.

Part of https://github.com/flutter/flutter/issues/160679

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packages/flutter/lib/src/material/carousel.dart

@@ -579,6 +579,10 @@ class _RenderSliverFixedExtentCarousel extends RenderSliverFixedExtentBoxAdaptor
 
   // This implements the [itemExtentBuilder] callback.
   double _buildItemExtent(int index, SliverLayoutDimensions currentLayoutDimensions) {
+    if (maxExtent == 0.0) {
+      return maxExtent;
+    }
+
     final int firstVisibleIndex = (constraints.scrollOffset / maxExtent).floor();
 
     // Calculate how many items have been completely scroll off screen.
@@ -641,6 +645,10 @@ class _RenderSliverFixedExtentCarousel extends RenderSliverFixedExtentBoxAdaptor
     double itemExtent,
     int index,
   ) {
+    if (maxExtent == 0.0) {
+      return maxExtent;
+    }
+
     final int firstVisibleIndex = (constraints.scrollOffset / maxExtent).floor();
 
     // If there is not enough space to place the last visible item but the remaining
@@ -673,6 +681,10 @@ class _RenderSliverFixedExtentCarousel extends RenderSliverFixedExtentBoxAdaptor
     )
     double itemExtent,
   ) {
+    if (maxExtent == 0.0) {
+      return 0;
+    }
+
     final int firstVisibleIndex = (constraints.scrollOffset / maxExtent).floor();
     return math.max(firstVisibleIndex, 0);
   }

packages/flutter/test/material/carousel_test.dart

@@ -1413,6 +1413,25 @@ void main() {
       expect(buttonPressed, isTrue);
     },
   );
+
+  // Regression test for https://github.com/flutter/flutter/issues/160679
+  testWidgets('CarouselView does not crash if itemExtent is zero', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: SizedBox(
+            width: 100,
+            child: CarouselView(
+              itemExtent: 0,
+              children: <Widget>[Container(color: Colors.red, width: 100, height: 100)],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.takeException(), isNull);
+  });
 }
 
 Finder getItem(int index) {