Fix more segfaults on NetPlay quit

Basically everything here was race conditions in Qt callbacks, so I changed the client/server instances to std::shared_ptr and added null checks. It checks that the object exists in the callback, and the shared_ptr ensures it doesn't get destroyed until we're done with it.

MD5 check would also cause a segfault if you quit without cancelling it first, which was pretty silly.

Source/Core/DolphinQt/NetPlay/PadMappingDialog.cpp

@@ -59,8 +59,8 @@ void PadMappingDialog::ConnectWidgets()
 
 int PadMappingDialog::exec()
 {
-  auto* client = Settings::Instance().GetNetPlayClient();
-  auto* server = Settings::Instance().GetNetPlayServer();
+  auto client = Settings::Instance().GetNetPlayClient();
+  auto server = Settings::Instance().GetNetPlayServer();
   // Load Settings
   m_players = client->GetPlayers();
   m_pad_mapping = server->GetPadMapping();