NetworkServer: initialize profilemanager_thread

StopServer guards the pointer and nulls it after the delete, but
nothing ever set it, so destroying a server that never started read an
uninitialized pointer.

NetworkServer.cpp

@@ -174,6 +174,7 @@ NetworkServer::NetworkServer()
 
     plugin_manager              = nullptr;
     profile_manager             = nullptr;
+    profilemanager_thread       = nullptr;
     settings_manager            = nullptr;
 }