I had a look at the code and found the relevant statement in Joystick.cpp
xpos[0] = (SDL_JoystickGetAxis(joy1, 0)-joysubx[0]) >> joyshrx[0];
I suggest that we can process xpos[0] further by
xpos[0] = (xpos[0] >> 1) -128
Then use an if statement to put all negative to 0