Fix Fix FP when using non-const members and also check return type of function for uniqueness

[pfultz2]

This should fix issue 8613:

class PortWindow
{
public:
  ~PortWindow();
  uint16_t newPort();
};

class CommandHandler
{
public:
  void f();
  PortWindow& port_window_;
};

void CommandHandler::f()
{
      uint16_t port1 = port_window_.newPort();
      uint16_t port2 = port_window_.newPort();
      if(!port1 || !port2) {
      }
}