fix crash when entering 000 and pressing +

Author: ws

lib/src/base_spin_box.dart

@@ -141,8 +141,8 @@ mixin SpinBoxMixin<T extends BaseSpinBox> on State<T> {
     _controller.value = _controller.value.copyWith(
       text: text,
       selection: selection.copyWith(
-        baseOffset: selection.baseOffset - oldOffset + newOffset,
-        extentOffset: selection.extentOffset - oldOffset + newOffset,
+        baseOffset: (selection.baseOffset - oldOffset + newOffset).clamp(0, text.length),
+        extentOffset: (selection.extentOffset - oldOffset + newOffset).clamp(0, text.length),
       ),
     );
   }

test/test_spinbox.dart

@@ -231,6 +231,14 @@ void testInput<S>(TestBuilder builder) {
       expect(tester.state(find.byType(S)), hasValue(21));
       expect(find.editableText, hasNoSelection);
       expect(find.editableText, hasText('21'));
+
+      tester.testTextInput.enterText('000');
+      await tester.idle();
+      await tester.tap(find.byIcon(TestIcons.increment));
+      await tester.pumpAndSettle();
+      expect(tester.state(find.byType(S)), hasValue(1));
+      expect(find.editableText, hasNoSelection);
+      expect(find.editableText, hasText('1'));
     });
 
     testWidgets('submit', (tester) async {