ExpressionParser: Clear expression variables on UpdateReferences call. I don't know if this is most sensible.

Source/Core/InputCommon/ControllerEmu/ControllerEmu.cpp

@@ -40,6 +40,9 @@ void EmulatedController::UpdateReferences(const ControllerInterface& devi)
 {
   m_default_device_is_connected = devi.HasConnectedDevice(m_default_device);
 
+  // Reset variables:
+  m_expression_vars.clear();
+
   ciface::ExpressionParser::ControlEnvironment env(devi, GetDefaultDevice(), m_expression_vars);
 
   UpdateReferences(env);