Fix overflow in scroll-to-bottom (#9389)

## PR Checklist
* [x] Closes #9353
* [x] CLA signed.
* [ ] Tests added/passed
* [ ] Documentation updated.
* [ ] Schema updated.
* [ ] I've discussed this with core contributors already.

(cherry picked from commit 1202f89)

src/cascadia/TerminalApp/TerminalTab.cpp

@@ -379,7 +379,7 @@ namespace winrt::TerminalApp::implementation
         co_await winrt::resume_foreground(control.Dispatcher());
 
         const auto currentOffset = control.GetScrollOffset();
-        control.ScrollViewport(currentOffset + delta);
+        control.ScrollViewport(::base::ClampAdd(currentOffset, delta));
     }
 
     // Method Description: