Don't let ConptyConnection be UAF (microsoft#4871)

I noticed a crash in debug builds when a connected application terminates;
we get its exit code, then we destruct, and the u16state is used after it's
destructed by us parsing the process's last words.

We should have been doing this all along.

src/cascadia/TerminalConnection/ConptyConnection.cpp

@@ -359,6 +359,10 @@ namespace winrt::Microsoft::Terminal::TerminalConnection::implementation
 
     DWORD ConptyConnection::_OutputThread()
     {
+        // Keep us alive until the output thread terminates; the destructor
+        // won't wait for us, and the known exit points _do_.
+        auto strongThis{ get_strong() };
+
         // process the data of the output pipe in a loop
         while (true)
         {