You could also add the subevent:
+ 'X Value' is less than 0
-> Set 'X Value' to 0
Or simply use the expression:
Player: Set 'X Value' to: max(Player.Value('X Value')+(Player.Value('Deceleration')*timedelta), 0)
Then you get the smooth floating point subtraction which never falls below 0. For small 'Deceleration' values, rounding may cause the movement to stop or become jittery, so you should prefer one of these methods.