WebContent: Tolerate invalid page_ids in ConnectionFromClient

When a tab or nested traversable navigable is closed, there might be
messages still in the pipe from the UI process that we need to
gracefully drop, rather than crash trying to access an invalid pointer.

Userland/Services/WebContent/PageHost.cpp

@@ -34,6 +34,13 @@ void PageHost::remove_page(Badge<PageClient>, u64 index)
     m_pages.remove(index);
 }
 
+Optional<PageClient&> PageHost::page(u64 index)
+{
+    return m_pages.get(index).map([](auto& value) -> PageClient& {
+        return *value;
+    });
+}
+
 PageHost::~PageHost() = default;
 
 }