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/Settings.cpp

@@ -276,9 +276,9 @@ GameListModel* Settings::GetGameListModel() const
   return model;
 }
 
-NetPlay::NetPlayClient* Settings::GetNetPlayClient()
+std::shared_ptr<NetPlay::NetPlayClient> Settings::GetNetPlayClient()
 {
-  return m_client.get();
+  return m_client;
 }
 
 void Settings::ResetNetPlayClient(NetPlay::NetPlayClient* client)
@@ -286,9 +286,9 @@ void Settings::ResetNetPlayClient(NetPlay::NetPlayClient* client)
   m_client.reset(client);
 }
 
-NetPlay::NetPlayServer* Settings::GetNetPlayServer()
+std::shared_ptr<NetPlay::NetPlayServer> Settings::GetNetPlayServer()
 {
-  return m_server.get();
+  return m_server;
 }
 
 void Settings::ResetNetPlayServer(NetPlay::NetPlayServer* server)