Handle invalid index in love.joystick, but then 1-indexed (bug #172)

--HG--
branch : minor

src/modules/joystick/sdl/Joystick.cpp

@@ -68,7 +68,7 @@ namespace sdl
 
 	bool Joystick::checkIndex(int index)
 	{
-		return index >= 0 && index < getNumJoysticks();
+		return index > 0 && index <= getNumJoysticks();
 	}
 
 	int Joystick::getNumJoysticks()